Trade routes over water?

Solution 1:

Yes, you can trade over water. First, you need to research Cargo Docks, available in Era III.

This allows you to build a city expansion of the same name, which must be placed on a water tile and adjacent to an existing city district. Once placed, the Cargo Docks will then create "ocean roads" that will be used by trade routes.

Do note that your 'target' city needs to also have a Cargo Dock in their city for a trade route to connect between them.

Solution 2:

To supplement DMA57361's answer:

Note that ocean and land roads connect - if you've got a landlocked city with Right of Way, next to a coastal city with Cargo Docks + Right of Way, it can make a trade route to a distant city with Cargo Docks (I'm not sure if the distant city needs Right of Way too, if it's coastal.)

The most difficult thing with trade routes is really goosing the AI into building the associated buildings; the AI seems reasonably content to at least get Right of Way built, but getting it to build Cargo Docks is an exercise in frustration at lower difficulty levels.

If you're planning to trade heavily with an AI, it's a good idea to go ahead and just gift them the associated technologies (or at least trade them), to get that impediment out of the way. Note that for obvious reasons, the Roving Clans are more inclined to prioritize these techs.