Until the advent of trade units in BNW, I have always been irritated by culture preventing any form of movement through coastal and ocean tiles. This is particularly irritating when you look at specific maps. For example, a Europe map. Every Europe map is terrible for naval movement. Getting through the Baltic past Copenhagen, the English Channel, Gibraltar...even getting past Sardinia, Corsica, and Sicily in the Mediterranean is nigh impossible without open borders or a DOW.
There really should be far less restrictions on naval movement. People complain about 1up on land, but what about coastal units? How can a player put naval pressure on a city with an ancient navy when the coast is literally one or two tiles wide?
No cultural expansion in ocean hexes is a great idea in my opinion. I also think naval units should pass through the borders of other nations freely. If you or another player doesn't like it, build more naval units and physically block them. That's how it was done in CivII. Dont want the enemy in your borders? Build forts and units to block them. Cultural borders inclusion in CivIII was an amazing idea, but after playing both IV and V I think the Devs should rethink cultural borders and how they can be better used, or even dropped, to better the 1up system.