Not all drypants are made equal. Many “dry” pants are only dry on shore, some are decent and are dry when you don’t go in above your waist, and then you have a few that with a good drytop will be very close to a drysuit (dry as long as you don’t swim something big and dry enough so that if you do it wont be dangerous, just slightly uncomfortable). By the way, in whitewater nothing is actually 100% dry… even a full drysuit will allow just a tiny bit of water in from the seals under enough pressure