I agree that boiling lobsters is cruel, shrimp probably die a lot faster than lobsters though because they're so much smaller and probably die more quickly in the hot water. If it helps any the scream probably had to do with the exoskeleton and escaping air like Pheasant said. Crustaceans do feel pain though, they have a nervous system.
Why all this talk about Treif here in the Torah section... Both Shrimp and Lobsters are forbidden for Jews to eat. They are both bottom-feeders and not Kosher.
I am not making a case that Shrimp and lobsters are in anyway kosher but I thought in the case of fish, if they have fins and scales they are kosher, regardless if they are bottom feeders or not?
Best and Warm Regards
Adrian Wainer