SCUBA-equipped conventional infantry is described in Tactical Operations: Advanced Units & Equipment (page 153, sixth printing) as having their weapons specially adapted for underwater operation. They can use any weapon they could on land as a result, albeit with the range halved.
Does battle armor equipped with UMUs and an anti-personnel mount also use this rule? If not, what are the rules for firing AP weapons underwater?
Battle armor without UMU capabilities may also find itself underwater, and under the rules in Tactical Operations: Advanced Rules (page 107, sixth printing) they may still fire weapons despite not being able to move. Could they use AP weapons underwater, despite not being designed for such conditions?