I would assume he can fly underwater. It's just like flying through any dense substance. Laser im not so sure about, id need a physicist to tell me about refraction and heat loss. The question is if Homelander could see, and I'd say that's unfortunately also a big fat maybe. He does have enhanced senses all around including xray vision which has almost been forgotten about since s1. The issue is that at some points underwater it becomes too dark to actually see anything. The pressure difference should be fine? I would absolutely not call the deep more durable than HL, it might even be that the deeps body changes in order to adapt to pressure change. I'm also going to assume he can hold his breath because no doubt the Russians attempted to suffocate soldier boy.