Since whales are teaming up with each other to take down yachts and teaching others how to do it I thought this would be a fun question.
If a majority of intelligent enough sea animals that could communicate with each other teamed up to mess with human activities in the sea who would win.
By the way for people that say that humans would obviously win we have already lost a war against emus before.
A simple hand grenade then. Shockwave from even small explosions are much more lethal in an incompressible medium like water.
So as long as we keep lobbing explosives in the water while staying out of it itself, the marine mammels don’t have a chance.