Need longer arms Jim ?

There are quite a few symbols already in DD3 and its add-ons that will work equally well underwater, or which can be repurposed to resemble something of that sort with a bit of imagination. Stone doors will work just as well undersea as on land, for example, while things like fungi groups will work nicely for any colourful creatures underwater - like small sea-anemones, for instance, especially if you make use of the varicolor options. But I'm sure Sue has this well-covered already!

The bridge shadows look fine on the ground, but not on the mesa tops, where they look like they're floating again, instead of the bridges attaching to the tops properly

Try moving the bridges Sheet just below the mesa tops one, and then copy the bridges onto a new Sheet above the mesa tops, but which doesn't have any Effects on it. That should still give you the bridge-shaped shadows on the cliffs and ground surface, yet still leaving the bridges looking like they're attached to the mesa tops.

And well done on finally chasing-away those irritating grey patches!

In case it may help Sue, you can find a moderate-res PNG of the seafloor map in the discussion on this blog from last April. The JG terrain symbols won't be much assistance, I fear, as they're the old dry-transfer rub-down-by-hand type (the map is from 1977, when this was state-of-the-art), but some of the information on it - and in the blog notes too - may be more useful.

Just been doing my usual after-the-event catch-up with this latest video.

Interesting to see the multi-level use of the software being worked through again, especially with techniques many will probably wish to emulate for their own, similar, maps, as well as highlighting some of the complexities of trying to copy large amounts of mapped items from one map to another this way. Things like the addition of new Sheets in the "wrong" order and without their Effects on the new level map are liable to catch-out many on their first trials like this after a clipboard copy & paste, so are definitely worth being more aware of.

This was brought into focus again for me recently with the multi-level Wyvern Citadel maps I prepared, which have just gone into the Community Atlas this evening. I was fortunate with those in one respect, because such maps have to be prepared individually for the Atlas, so I could just save the "old" map and then start the new one from a fresh version of that, which kept the Sheet Effects intact. Of course, even this isn't problem free, because you still have to move around the order of those Sheets that are now at a lower level than the one you're working on, and create new Sheets and their Effects for the fresh items as they're being added!

From a long-time GM's perspective, what I'm looking for in a map I'll be using to run a game is something clear enough that I can find the information I need immediately, and if necessary on-the-fly. Fancy artwork's all fine and good, but if it takes me ten minutes to work out where all the entryways are to this room/cave/building/etc., that's not a good GM's map to me!

That's not to say a map can't be good-looking as well as practically useful. From the very first D&D maps I drew, back in 1977 or so, I've always used colour and shading as additional GM's visual aids. On published scenario maps going back to my first Judges Guild products (the earliest available for D&D, basically), the ones I used, or hoped to use, are all hand-coloured, so I could tell at a glance just what - or who - was where. And it's great to have the assets of CC3+ to play around with now when constructing any maps, of course.

Where a map might also be used as a visual aid for players though, up to and including a tabletop battle-map, something more artistic would be preferable, to help give the setting more flavour. In later times, I sometimes used 3D cast or papercraft scenery this way as well. With 3D cast minis, it makes sense, and helps folks get a better idea of scale, I think. This, of course, is not a cheap or quick option!

One thing I did find in using 3D terrain was it helped to have a GM's map, and then a much simpler sketch-style map so I knew what bits of terrain needed to go where, to speed up the tabletop construction process, and I developed this in CC3 (as it then was) using the simpler option of darker (sometimes coloured) lines on a white background, as it didn't need anything fancy, being just for my own reference.

Looks interesting Sue, and it'll work nicely as other types of shallow-water sea vegetation, I think.

I'm not overly familiar with eelgrass, so am having to judge by online images only, but this looks a little large and robust for those types that have come up via Google so far (where there's anything to help judge size, at least, and I'm assuming "your" barrel's about 2-3 feet in diameter). Not sure how representative a sample that may be, however!

When I wrote here to present details about my first land-based contribution to the Community Atlas Errynor in northwestern Alarius, I noted that 2,000 x 1,000 mile map had behind it a collection of forty 250 x 200 mile sub-maps, so that in essence, the main Errynor map was a summary of the major points from those 40. I also noted the plan was to present the 40 sub-maps separately over time for the Atlas. This map is the first of those - at last...

My initial thought was to prepare the sub-maps for each corner of the Errynor map, because those would encompass aspects of all the chief landforms, including the undersea areas which had been one main element in determining how the Errynor map was drawn. Wanting to try something different to that main map, I opted for the Herwin Wielink style, as that includes a number of useful options for the range of terrains needed.

However, like most CC3+ style packages, it largely ignores the undersea environment (as discussed in this Forum topic). So I knew from the start I would need to create some fresh symbols for that, though sadly I'm no artist! Needs must, though. Numerous problems beyond anything map-related meant progress with these was very slow, so it was only in February 2020 that those new symbols were completed:

These were all tinkered together in GIMP, incidentally.

Knowing from the outset that I couldn't create anything with the artistry of HW's symbols, I instead drew mine taking aspects from that style only, such as the building bases having a similar fading "looseness" to help blend better into the underlying surface fill textures, and having the building shadows fall on the same side, for instance. Due to the size constraints on what symbol files added to the Atlas can be, I opted for just this very limited number, as I knew others already in the HW style could be reused for additional types quite readily. Of course, as we've discussed repeatedly on the Forum previously, further variants and extra items could be added almost ad infinitum!

OK, this is a bit more "tropical seas only" than I was thinking.

The coral overhang is, as far as I recall, only a thing with living corals, not the underlying rocks (which themselves are likely to be largely made up from increasingly compacted dead coral skeletons). The overhang will only survive as long as its own weight/strength and tidal action allows, of course, so collapse of unsupported areas is still liable to happen, especially when there are no longer living coral polyps there to help keep the whole "glued" together. As you'll gather, I was assuming the rocks were "just" rocks, not intended as the immediate substrate for living corals!

Keeping the coral symbols separate from the rocky bit ones would offer greater flexibility, certainly.

The wildlife looks absolutely brilliant Sue!

Not quite so sold on the geology yet though. The above-sea sand seems a tad too bright, and the white outline looks odd - as also on the surfaced rocks. I think I can see why you've drawn it like this, but actual rocks, and sand, tend to get darker when wet, if sometimes only subtly so, so the line (actually a zone whose size is determined by ripples splashing on the exposed surfaces, were they real) should be darker where the rocks/sand meet the sea.

The stacking of the rocks doesn't look quite right either. Higher rocks should lie at or within the outlines of those below them if supported by only a single rock, as anything else wouldn't be stable, and would simply collapse into a different pattern ordinarily.

Not sure if the rocks are symbols or simply drawn polygons however; if the latter, the stacking problem is easily resolved, and I'd imagine the water-line effect could be handled with an appropriate Inner Glow Effect. If they're symbols, though... sorry!