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Namor was really giving a lot of credit on himself and his technology to swear revenge against the surface world, but the only good advantage he had was that he's basically Hermes and that fact that half the earth is made of water. But the only thing him and his people had going on, is some water bombs and some dope fighting skills, while the earth is literally being guarded by at least 50 heroes, that consists on literal gods, mages and people that can bend the universe at will.
Really, all it takes to win this war would be to Thor send an thunder on the sea and there it goes! Half his army is gone! We're eating fried shrimp on dinner!