i would like o have more explanation about the theme. for example can i make a game about fishing and selling fish in naval ports, fighting pirates in sea ,. can i just make a game about modern or medieval naval warfare

You can make whatever game you want. unless you can connect the game to the theme "Aqua". You have a nice idea though and yeah your game fits the theme :]

The idea is you can do whatever you want with it. If you want to introduce 'aqua' as naval warfare, that fits! It's there for creative spark.