All the map templates prioritize land battles, and make navy units almost useless

[Moginheden]

All of the map templates that come with the base game create landmasses that you share with other civilizations. You occasionally have to cross a couple of water tiles to get to all the opponents, but in general the battles happen in a very land-locked manner well out of range of naval units. (I don't own the expansions yet so I don't know if this was fixed in any of them.)

I'd like to see a randomly generated template that contains lots of small islands where naval battles are the primary focus. Most cities should be in range of a battleship on this template.

I've made a map like this in the map editor, but I'd like randomly generated ones rather than playing the same thing every time.

 

[Scrum Lord]

The R&F expansion added an Archipelago map type, which is more naval based than Island Plates. As far as the base game is concerned, that's probably your best bet at the moment.

On the other hand, you could download a free custom map script from either the Steam workshop or from civfanatics. The latter has an entire section of mods devoted to maps and map scripts.

 

[Moginheden]

Do the map scripts create random ones or just custom static ones?

 

[Scrum Lord]

Do the map scripts create random ones or just custom static ones?

Random ones. Just like the base game's non-static map types (Continents, Fractal, Pangea, etc), which are also map scripts.