This is different from saying "Wow, they must drink a lot", right? This should be וואו, וודאי שהם שותים המון, or something like that, right?
It is different as you assumed. I'd say "הם בטח שותים המון". More formal perhaps, and very close to what you wrote, is "הם ודאי שותים המון". The sentence "ודאי שהם שותים המון" means "of course they drink a lot".
Does the "must" here mean only "they need to drink" or also "based on what I know, I believe they drink"?
עליהם לשתות חרובֶה Is my answer completely wrong (other than the
?חרובֶה mispelling of