I realize this thread is over a year old, but it has relevance to my first winterization of my 2009 H36 this year (I paid someone to do it last year). I'm wondering if anyone has solved this, or has a better solution. FWIW everyone hit on the solution - open the seacock, and open the faucet, and the pump will recognize the demand and start pumping.
I suspect I have the same washdown system as Marie - a spigot and rotary handle in the anchor locker next to the windlass in the bow. The pump is just forward of the water tank, under the V berth in the bow. The rotary handle is something of a mystery - it has no stop and can be continuously rotated in either direction, although in some position(s) the saltwater is reduced or stopped, in others it is partially/fully on. The saltwater connection is teed off of the hose for the air conditioner (I found out the hard way you need to leave that seacock open even if you can't run the A/C off of 12v, or you don't get any washdown!!) There is no on/off switch or breaker that I can find for this pump. It works as a demand pump, such that when the spigot is opened with the rotary handle, the pump actuates until the rotary handle is placed in the right (non-obvious) spot and demand flow is at zero.
The problem I ran into during winterizing was that when I disconnected the washdown hose from its Tee off the A/C unit and pumped antifreeze thru the washdown hose, the pump was unable to re-prime because the thru hull for saltwater is shut off, and there is air in the washdown line created when some antifreeze drained out before I reconnected the line to its Tee. The pump runs continuously trying to get up to pressure, which it can's with air in the line, and I can't flush out the air without opening the seacock (and ruining the winterization). It is virtually impossible to disconnect the hose connected to the seacock and winterize both systems that way because it is a short and very stiff run. I am impressed they got it on there - probably only possible when the hose was new. So, my only choices were to shut off the 12v system (no thanks), or cut the power (literally) to the pump, so I snipped the wire. Problem solved until Spring.
I'm thinking of installing a simple on/off switch in the V berth so for winterization I can just shut the pump off manually after winterizing, then turn it back on in the spring when I purge the antifreeze and get all back to normal. Alternately, I can install something to make winterizing the a/c and the washdown system easier that doesn't involve disconnecting hoses and losing prime. Its probably a PITA to redesign the seacock-to-A/C-washdown plumbing, and i'd literally have to cut stiff and stuck hoses off to make it happen, but I have this winter on the hard to figure it out. Would love some ideas.
If you read this far, thanks.