The HEV suit being what it is, Gordon can't feel nearly as much brushing past. Just that there's something there and that it's probably not a fish, considering these waters. More likely-

Gordon is a scientist at heart and has been a thinker all his life. The fact of the matter is, though, that at Black Mesa he saw that first scientist gunned down by the Marine supposedly there to rescue him, realized that he was going to die within seconds himself, and acted on that information before his brain had time to process it in anything like a conscious thought. This? Is like that. The syllables saltwater crocodile don't even have time to form in his brain. He just holds up one hand very briefly, assuming Voodoo can even see it- and then dives.

If he ever sees Hephaistos at the Bar again he's going to thank him personally for the work the Engineer of All Engineers did on the suit's helmet. He's down there a while, a fact well attested to by the thrashing, roiling waters in the immediate vicinity.

Some time later he breaks the surface again, crowbar in hand.

There's still stuff stuck to it. And the water is considerably darker now than it was a short time before.