Maybe because Superman can’t breathe under water? Or maybe they are so far under water that sunlight doesnt reach where they’re fighting and Superman would become underpowered?
not being a smart ass, just thinking of a reasonable explanation.
Superman can survive in space, so he doesn't need oxygen
And he doesn't lose his power when not in direct sunlight, he's more like a battery that uses the sun to charge his power levels
Clark: Hello?
Arthur: Hey Supes, I could really use your help!
Clark: Hey Arthur! It's always nice to hear your voice! What's up?
Arthur: A major threat is trying to kill me and my family! Can you come deal with it?
Clark: I wish I could, but I'm swamped at work! Old Man White has been really riding my ass!
Arthur: Seriously?
Clark: Yeah, I'm terribly sorry!
Arthur: You literally could've handled it in the time it took you to say you can't do it
Clark: Hello? Hello? Sorry, Arthur, you're breaking up!
*Clark hangs up*
Arthur: I know you can still hear me!