Don't worry about the layer it's on, the sheet it's on is more important. It will put it on the Grid sheet and that can be moved, and it's placement will determine how things are rendered. If your land renders before the sea (appears higher in the order of sheets), you can put the Grid sheet in between.

If the sea renders before the land, there are other tricks I can suggest, like maybe a color key cutout (have to think through that), but others may have ideas.

Lots of great details. On this eve of All Hollow's Eve, sending my thoughts to JimP and his loved ones.

I am working on the below-ground caverns for the lair. There are five levels, but three of the levels will primarily be occupied by a massive central cavern, with just side tunnels on different levels. They use Creepy Crypts as a base.

Here's the topmost interior level. The wider, smooth tunnels are lava tubes, with additional tunnels carved out more roughly. (Steam vents courtesy of Marine Dungeons.) I was going to use a gray marble for them, but the reddish ones from Marine Dungeons provided a nice contrast and a break from the mostly grayscale map. The lighter area towards the center bottom is below the chasm from the above-ground map, open to the sky.

Here's the middle, looking down at the lava. There are layers of Bitmap 20 transparencies to shadow between layers, which will be removed as we descend to lower maps. There are two sheets of lava, with the darker Lava Flaming from DD3 on top of the brighter Lava Molten, with the Color Key effect and an edge fade used to allow the brighter lava shine through in patches. Gives it some texture. The two lava types are also scaled a little differently to help prevent pattern repletion where there brighter lava shines through.

I love the cliffs from Forest Trail, but I wanted something more jagged. I ended up using the cliffs from the Japanese Temple annual, placing them on the Symbols Clifftop layer so that they fuse nicely with the cave walls.

This version connects the central island with a natural bridge:

But I'm considering using a constructed bridge from either Darklands City or DD3.

Darklands City

DD3

Here's a practice attempt at creating the exterior, above-ground areas of the volcanic Womb of Ashes. I did a lot of experimenting with different fills and techniques from all over the place, so there's a lot of clutter in the FCW file. When I'm happy with the results, I will create a fresh file that just brings in what I need.

This has a Creepy Crypts foundation with extra fills, symbols, and sheet effects inspirations brought in from Forest Trails, DD3, and Spectrum Overland for the volcano.

The terrain transparencies are from DD3. I love the dirt patches from Forest Trail but I really needed ones that were gray, not brown, and as I surmised, I couldn't change their color or use the RGB Matrix on them, even after exploding them. Creepy Crypts also has different terrain patches, but they're in darker shades of brown, not black/gray.

For the final lair, the volcanic Womb of Ashes, I wanted to have some sigils engraved in the floor where the final ritual is being performed by all the covens united as a Grand Coven of 13: the Drowned Sisters' Grotto (sea hags), the Withering Glade (forest hags), the Miremothers' Den (swamp hags), the Frostmaidens' Cradle (alpine hags), and the Arch-hag at the Womb of Ashes. (Plus, it gives me an excuse to once again use the Marine Dungeons brass inlay that I love so much.)

I had previously discovered that some of the varicolor symbols used in CA15 Heraldic Symbols (2008) can be set to the magenta color to work as an inlay. I was delighted to discover that some (not all, but some) of the vector symbols from the overland map sets could also be exploded and turned magenta to do the same.

So...here are my sigils for each coven. The Forest Hags' was the easiest -- it comes exactly as is from the Marine Dungeons annual. Originally I was going to use the symbol of the tree without the runed circle, but then I realized that the runed circle could tie them all together and strengthen a visual similarity. The shell for the Sea Hags' symbol also comes from Marine Dungeons. The snake for the Swamp Hags' sigil comes from CA15. The water waves, swamp grasses, mountains, and volcano all come from CC3 Vector BW overland style that comes with CC3. The fire symbols were ones I made for one of the Trial of the Elements map in the Atlas. I couldn't find snowflake images that I liked, so I designed those ones myself specifically for this.

I thought I was going to have to draw a lot of stuff myself, which is totally not my forte. Really delighted to find that some of the vector images could work for this.