This is what I meant before:
Sailers can sail above it, land units can't enter, aquatic and amphibious units can enter. I didn't check if non-swamp-survival amphibious units get swamp terrain penalties, but I quess they do.
With this (and special sites):
Coastal Plane: Impossible.
River: It would be nice if rivers could be defined somehow. There are large and small provinces; perhaps rivers should always be large?
Shallow Sea: coastal seas?
Coral Barrier: can't stop sailing, but sea/forest/farmland?
Sea Basin: also known as sea?
Confluence: Very high chance of Holy and Water Sites, average otherwise - this is interesting. Something similar might happen if the sea had the Freshwater tag? Possibly it would just slightly boost income and allow "There have been floods and XXX people have drowned" events.
Deep Trench: Also known as deep sea? Alternatively, also known as Gorge of Glowing Wish.
Sargasso: (forest of kelp) High in Resources, Supply, low in gold, sites, except for Nature sites, which are high.
Sea Cave: Sea + Cave could be done, but sailing would be allowed. Maps could be constructed so this wouldn't matter (e.g. sea/cave province connecting two seas with each other, with no or at most one land neighbour)
Tumulous: Can't stop sailing, sea + border mountain
Salt Marsh: Swamp + Sea
Seamount: Mountain + Sea