I'm also not sure what problem this is meant to solve. There are only two real underwater threats: Amphbots, and submarines.
Submarines are an expensive raider that specifically uses the stealth of being underwater to fight things. With long range sonar they'll just get completely bodied by ducks and hunters.
Amphbots pay for being underwater by being
slow as hell. It's not free for either of them.
Fundamentally, being underwater is the only real terrain masking afforded to sea.