Yeah, drums are like that. Water fills the drums and you have no brakes, then kinda washes them clean and makes them grabby or continue to slip depending on your pad material. Also like Mr Rose says, water bothers bad cables too. New shoes and cables are less than $50us last time I bought them.
I run front drums on both my fast Blasters. Not my first choice but they work OK. Disks are much better but I can haul to a stop reliably, and the drums were very cheap to repair to service.
Years ago I did work on many drum systems on off-road trucks and bikes, drilling vent holes and cutting water shedding slots in the shoes and drum. It all works but I find the Blaster works pretty good if I test my brakes a couple times as I leave the water. A couple good pumps and they purge the water.