First thing you think of when you hear "Armored Core"

[seraquiem]

They're probably not waterproof, kind of like a car if it's submerged in water. They're not Anno Domini Gundams which can fight underwater XD

Technically all Gundams have the ability to fight underwater (unless there's some exception I do not know of), to varying degrees with proper equipment and design. But the GN Drives do give CB Gundams the advantage in underwater combat in terms of propulsion. Likewise, their beam weapon power, like all other gundams in all other universes, are dramatically decreased.

ACs probably cannot withstand the water pressure. If you see some AC design sketches and model kits, the distance between the outside of the core and the inside of the cockpit is rather small. Yet again, those design sketches only show lightweight and midweight cores. The heavyweight cores are obviously a lot thicker.