When using IO (considering it's supposed to introduce lots of technical challenges), I would prevent all vanilla thrusters to work in water, because IO adds specific thrusters (conventional rotor blades) that are supposed to be used only in water.

But this is specifically about IO anyway.

Same reason as hydrogen really "Game balance" what reason would you ever need a propeller if a much faster and cheaper thruster is available. I almost disabled atmo thrusters for the same reason but thought that atmo would be for planets and hydro/rocket thrusters would be for space. I think of it as the hydro/rocket thrusters are not designed for underwater use so it turns off.

Why wouldn't a rocket engine work underwater?