If Tinkerer foes plan to ever introduce a Mermaid character to the game then I would say he should do it ether in the N-West of the map or the N-East. If he has an inland sea then that would be a saltwater area for the mermaid to live in.
N-East: If he dosent want to create a new big body of water then he can just have the mermaid living in the large lake/sea that surrounds the Tower of Dread. Mabey have the map open further east past the are with the bridge.
N-West: If he wants to create a new area then he could put the seaside area of an Inland sea here. The Great Sealake or something. Then he could have new enemies that patrol the beaches like giant crabs or gillmen/deep ones. If he wanted to do some Lovecraft parodies then the gillmen/deep ones could worship a squid god...
I dunno. Might be fun if he wants to introduce a Mermaid to the game. Obviously this is neither confirmed or denied so just speculation & ideas from me.