Reid used DIVE! (to show you how I would use the tiles you see below)
Do you have giant parts of open sea on your world map? You want to make something out of them?
Now you can!
I have to say, those ledges can be fiddly to map with, I recommend shift mapping of just the parts you need so you can get more mileage out of a layer.
You can technically add a lot of the smoother overworld mountains here, too, and I guess a lot of ruins would do great on the bottom of the sea, as well.
For that extra bit of fun I added some black smoke to come out of those black smokers ;)
So, where the treasure lies? Legends say, south of the whale skeleton, somewhere within the algae forest near a sunken ship...
Wheather or not you add a blue tint or add a water filter is up to you.
Also I wanna recommend using "tiny" VX/Ace sea creature sprites as giant overworld things, like those here.
Spoiler
Or add a (in comparison to the map) giant anemone or some fun bubbles.
You can also use Guardinthena's whale as a giant godlike creature (could also lead into the flesh dungeon, if you remember those tiles...)
Rasak's Jellyfish and school of fish are are small enough to work on a worldmap, and the clam could also be a massive goldlike creature!
Some of whtdragon's sea life will also work well, like the mini manta's becoming maxi ones, the school of fish and more possibilities for sea godlike creatures to meet underwater.
(I am usually working on the calendar posts immediatly after posting and during the day, so I am probably not able to answer them well, but be assured I read and love them all!)
And last but not least: I struggled so hard to put things on B, cause there is just not that much you can actually add underwater, especially on a world map scale. Testmapping showed the items I got stretch really far, especially with the elevation already making the maps more interesting than just plain flat ground.
What would you have put on B?