Humans.

We can be “not at sea” and still kill sea life, but dolphins can’t get on land.
We’re currently trying not to kill sea life and doing a pretty poor job of it. If we were trying we could do a number on marine mammals just by having the navy use their crazy powerful sonar all the time, instead of “minimally”.
“Ah, you’re trying to boop my ship. Have you considered having your inner ear destroyed so you can’t tell up from down, use echolocation, or communicate, resulting in a horrible death by drowning or beaching?”

Humor of it aside, the emu was was an attempt to kill the emus eating farmers crops where they used military resources to try to do pest control for people given vaguely subpar farmland. Turns out that stationary machine guns are not the best way to kill emus. After a few attempts only killed a couple thousand, they switched back to just having hunters do it and that got tens of thousands.
Later, they just used “fences”, which proved insurmountable to the emus, which were forced to just walk around the farms instead of cutting through.

I know this is just being silly, but stop and think about the difference in scale between a yacht and the larger ships in the ocean. There was a recent case of a cruise ship showing up at port with a whale over it’s bow. The ship rammed a whale, effectively beached it on the bulbous bow and no one on the ship noticed. And then you have things like an Eisenhower Class Aircraft Carrier. At over 1000 feet long, 250 feet wide and displacing over 100,000 tons the sheer scale of the ship is hard to imagine without seeing it. Imagine taking a sky scraper, tipping it on it’s side and floating that out to sea. That’s what we’re talking about. You could have 10 large blue whales laid out tip to tail along the length of the carrier. An entire pod of whales ramming such a ship would just result in whales with concussions. And then CWIS goes brrr….

Whales, dolphins and other marine animals are amazing, but their scale and coordination pales in comparison to what humans do. We have basically no natural weapons or advantages in strength or speed. But, we dominate the planet because we can plan far ahead and work in groups much larger than a local tribe. We also harness energy in ways well beyond what animals do. Even something as simple as fire is outside the ability of other animals to create and use effectively.

Use sonar loud enough and it’ll kill or at least damage pretty much every marine life it reaches, so humans win the battle. But lose the war because us winning just means mutually assured destruction.

Alfred Hitchcock’s The Fish

the only reason those whales are surviving is because they’re legally protected and people don’t want to hurt them. we hunted whales nearly to extinction without even using gunpowder

Humans have a history of eradicating species without even trying. If we put our mind to it we’d have the oceans scraped clean by next week.

Free Willy, with every purchase of $20 or more

Things like tanks, guns, and flamethrowers aren’t going to work in the water. Even just the dolphins could take humans down. It would be like the emu war, just without the weaponry and with the enemies being smarter.

Pretty sure I can punch a penguin in its stupid face.

Can’t guarantee if I could survive the following fight, tho.

At sea? I would not even fight. The sea creatures rule the sea.