Hulk can breathe underwater, but he could not originally accomplish this task. Hulk can breathe in outer space, but it also was not always so. These skills were picked up over time when his body was traumatized by drowning or suffocating in outer space and it mutated to allow his survival.

Can Hulk survive underwater?

HE CAN BREATHE UNDERWATER

With his enhanced strength and massive size, the Hulk is also able to withstand enormous amounts of water pressure, and as a result he's been seen on the ocean floor on more than one occasion.

Can the Hulk suffocate?

Although the Hulk was in his gray (and arguably weaker) form, his lungs managed to hold enough oxygen to keep him from immediately suffocating in the vacuum of outer space.