Asklemmy

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.

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.

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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.

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

By the way for people that say that humans would obviously win we have already lost a war against emus before.

Also, a crusade against sparrows ended poorly for us, too, though it was an indirect loss.

There's even a band called Red Sparowes that released an album called 'Every Red Heart Shines Toward the Red Sun' with song titles explaining this:

• "The Great Leap Forward Poured Down Upon Us One Day Like a Mighty Storm, Suddenly and Furiously Blinding Our Senses."
• "We Stood Transfixed in Blank Devotion as Our Leader Spoke to Us, Looking Down on Our Mute Faces with a Great, Raging, and Unseeing Eye."
• "Like the Howling Glory of the Darkest Winds, This Voice Was Thunderous and the Words Holy, Tangling Their Way Around Our Hearts and Clutching Our Innocent Awe."
• "A Message of Avarice Rained Down and Carried Us Away into False Dreams of Endless Riches."
• "'Annihilate the Sparrow, That Stealer of Seed, and Our Harvests Will Abound; We Will Watch Our Wealth Flood In.'"
• "And by Our Own Hand Did Every Last Bird Lie Silent in Their Puddles, the Air Barren of Song as the Clouds Drifted Away. For Killing Their Greatest Enemy, the Locusts Noisily Thanked Us and Turned Their Jaws Toward Our Crops, Swallowing Our Greed Whole."
• "Millions Starved and We Became Skinnier and Skinnier, While Our Leaders Became Fatter and Fatter."
• "Finally, as That Blazing Sun Shone Down Upon Us, Did We Know That True Enemy Was the Voice of Blind Idolatry; and Only Then Did We Begin to Think for Ourselves."

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

Alfred Hitchcock's The Fish

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.