One way to check for dehydration is to gently pinch the skin of the guinea pig's back. If the skin snaps back quickly, then the pig is likely not dehydrated. If it goes back slowly or doesn't at all, then the pig is probably dehydrated. If the pig is dehydrated, I suggest getting it to a vet right away (plain pedialyte is sometimes used as supportive therapy, consult a vet). Dehydration is very dangerous for little animals like guinea pigs.
Vegetables have a high water content as do fruits. I would suggest romaine lettuce and bell peppers, as they are good staple vegetables according to
Ly's chart, and also have high water content.