Why do I get the feeling you're not talking about Terror From the Deep?
Terror from the deep, i hated that game as much as i loved the first one, i don't beliveve it's possible to ironman this one on veteran and above, those damned crabs arrive too early, or maybe you just ignore the missions where you know they'll show up with some practice ?
I never got a lot of practice, without the proper weapons, those crabs are likely to make a carnage, really a huge change from the first game, even the crysalids from the first were far easier to manage.
Not only those crabs have an insane armor but they hide in boats compartments.