People always say that sea battles are useless, because the people on the ships are always stronger than the cannons.
But a lot of the times the people standing on the ships are devil fruit users, not to mention that ships are the main mode of transport for everyone.
Protecting their own ship and sinking enemy ships are still important. The truth of the fact is that sea battles are only guaranteed for weaklings, as they don't have anything stronger at their disposal, like common pirates and marine scrubs.
Msybe this could've happened if One Piece was a more realistic series and not Shounen.