Doomsday Naval Base

Because some countries are landlocked, so naval bases would provide an unfair advantage to some.

ok...but then why do countries in doomsday (even landlocked ones) have so much completed naval research?

also should you build naval units in this map?

Because the research is available to everyone, but the bases are only available to sea bordering countries. You can build naval units if you want, my personal preference is to focus on land and air first.