alien halo

Actually he can survive for at least 90 minutes under. He would actually suffocate, not drown (r/wooooosh moment)

It must be the work of aliens...

Maybe his armour electrifies him when he touches water

Sir, I have a status report on the armor. It has self-recharging shields, a swappable back slot, and looks badass... But there's a problem.
What is it?
Our advanced technology can't come up with a better way to create an invisible barrier to keep you on track, so your suit can't go into water.

But small puddles and rain is okay?

lol, yes.
Go ahead and play in the kiddy pool, Chief, only large bodies of water affect you.