There were some ideas in the forum I would like to relate. There was a suggestion for a new water tile, the delta. Basically if its a river + coast then there would be an extra X

, X

, and X trade route bonus. I think it adds more flavor to water tiles.
I've always pushed for the addition of the "River Port" which is a building they have in the
Rise of Mankind Mod. Considering historically how big some inland ports are (St. Louis comes to mind) this would be a nice thing to have, especially for those who like to play the Erebus map that has long rivers.
An idea that I'm not stealing from others is a new water monster, the giant spikefish, or shark or something. A much weaker unit than turtles and sea monsters that appear early in the game and would be an animal instead of a beast. That way the seas aren't so deadly early in the game (because there would be no turtles and sea-monsters) and there still is a chance of danger. Plus you can upgrade sea units from the start of the game.
I totally agree! The early sea critters are far to nasty. Plus, unlike the land, there is always lots of Fog of War so sea creatures will spawn the entire game so having them get nastier makes sense.
Plus, two comments about Cultists or maybe Luann.
First, one of my favorite units from the old Civ III Rise & Rule Mod (predecessor to the Civ IV Rise of Mankind Mod) was the Polynesians had a out-rigger unit really early. It could go anywhere, coast, sea, ocean but if it ended it's turn in a sea or ocean tile, it had a chance to sink. It could hold one unit but it could be of any type. It was a great unit for finding and colonizing islands before anyone else could. I think it would be a good unit for Luann and maybe for followers of the OO.
Secondly, about Cultists casting Kelp. I think ships should be able to get some sort of animal capture ability and then if a shark or giant turtle is capture, you could cause it to create some sort of marine resource sort of like how capture animals can be made to create new lairs.
I'd also like to reiterate my long held belief that when a ship is attacked by a pirate (or a pirate is attacked) that there is a chance a 'prize ship' is created. These ships wouldn't be very good ships, but when brought back to port, would act as 'super slaves' in the way of hammers when sold.