They drink...you're just likely not catching them at it. Remember bloods are mostly sedentary, and very much nocturnal, so you may be asleep when they're actually out & about and doing their thing.

Soaking may help, especially if you're treating a snake with any sort of dewormer. Keeping the animal hydrated is essential during any type of medical treatment.

Keep in mind that bloods may occasionally be picky about water freshness, too...I change waters 2-3x a week for my gang, just to appease any of the divas in the group.