Thanks for the advice. The hose was kind of stiff so maybe the answer is 3/4" Bilgeflex. I had gone with something sturdy in case rainwater froze inside.
Also it turns out Rule didn't like the idea of a check valve, though I don't see the point of pumping the same water over and over.
Somebody has probably fitted a seacock to the sink drain through hull fitting. If so I'd like to hear it. I've heard that in surfing conditions you can get a stream of water to shoot up through the sink drain and hit the ceiling (though I've never actually tried it).
