I noticed on my play troughs that ships are not used as much as ground units or planes, problem is that ships become absolute on maps that dont have much oceans, also there is no much use for a ship unless you want to take over a dock or do a sea invasion, but for sea invasions people prefer to use planes as they are a quick way to transport troops safely.
My point is that for us to be able to use ships we need to have maps with more and bigger oceans, we need ships to be a key weapon as for to be able to transport more troops and be faster than planes, because in every map there is little to no ocean.