I decided to replace the hoses from the cockpit drains and the sink today that attach to the molded in through hulls. I found a $2.50 plastic check valve spliced into the hose between the sink drain and the scupper pipe. Makes no sense to me that one would splice a low pressure, home use piece of plastic into a piece of high pressure heater hose intended to save my boat's bacon if the the water in it ever froze. I'll replace it with a single length of hose, no check valve.

I thought I'd offer this as yet another reminder to never assume the previous owner of the boat got anything right. I should have found this a few years ago but the part of the hose I could see and the clamps appeared to be in excellent shape so I hadn't looked further down until today.