Have been away and insanely busy with RL, but considering that I have been wanting to do an ocean-based campaign for a while this is THE CCC theme for me.
I'm particularly interested in possible tileset modifications and additions (don't know if these would qualify for the CCC though):
Addition of animated 'currents' within the underwater tileset giving the impression of 3D movement (essentially this would be same old raise/lower, but with an illusion of swimming up and down in the middle of the ocean depths.
'Walkable' ocean water that you could put next to ships on which various sharks and sea monsters appear to swim around (and perhaps even ships too, if these were converted to creatures for the effect of sailing about (though anything up close would have to be a tile, obviously).
Proper swim animations for humanoids (and possibly even for quadrupeds, but that's just dreaming...)
Animations (or even just more different static versions) of ships/ship tiles with sails being unfurled or taken down. So you can have same ship model - with & without sails, stuck on a sea-rock or otherwise wrecked or at the ocean bottom....
Giant kelp forests - either as tiles or a selection of large-ish placeables.
Icebergs and more types of sea ice.
More brainstorm results forthcoming, but run for now I must. 
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p.s. some amazing stuff in this thread already!
