Like I said in my post about super-swimming, Earth is 70% water, but the interesting things happen on the other 30%. That said, it's not like it's empty. For example, here's a short list of things which we've dredged up from the sea.
- A colossal squid, 43 feet long, and around 900 pounds.
- A Portuguese man-of-war, that can reach 30-150 feet with its tentacles.
- A fin whale, 80-90 feet long and weighing tons.
During World War II, the US Navy did experiments, along with the New Zealand military, into creating a "tsunami bomb" under the name Project Seal. The goal was to devastate coastal cities - which frankly is most of the important ones. Humans may not live in water, but we live near it. As Magma pointed out to me once, "there's been a city at the mouth of the Tiber for the last 10,000 years".
The program revealed that a line of explosives, about 8 megatons and 5 miles off the coast, could indeed create a tsunami. Most of the energy would be wasted hitting the continental shelf, so it was considered inefficient. But hey, you got a better idea?
The United States' Ohio-class submarines (fourteen currently active) carry Trident missiles, with eight W88 warheads. Each of these has a yield of 475 kt, so you need two such missiles to reach 8 megatons. More missiles means more explosions, and each Ohio carries 24 Trident IIs.
Remember what I was saying about giant sea monsters underwater? How hard do you think it would be for someone like Sea-czar to find and take control of an Ohio-class sub with his army of aquatic minions? The real barrier at this point is a knowledge of submarines, nuclear engineering, and hydrodynamics. But once you can do that, a single submarine lets you threaten every city in America, simultaneously, staged from depths that'd prevent anyone but the Navy from acting against you. And even they'd have a rough time.
So yeah, I'm just going to say this one more time: respect aquatic supers.