One more thing...

[Belanos]

You need to differentiate between actual Ocean tiles and inland water bodies. I noticed in a game that since I had a lake next to a city I could still build ocean vessels, even though they'd never be able to go anywhere.

[greatUnknown]

IIRC this happened in SMAC also. There, however, the problem could be resolved ultimately by
lower the terrain to create a channel to the nearest ocean. This ability doesn't exist here.