How soon is too sonn?

It all depends on how cold it gets and for how long. If your boat sits in an unheated garage you're probably good to a much lower temp.

Outside it would take a hard freeze(low to mid 20's) for several hours to do damage. If the temperature barely dropped below freezing for a couple of hours at night the engine won't have time for the water in it to freeze. However if your weatherman in Washington is anything like they are here in Iowa they may predict a low of around 30 and it could actually drop down to 20 or so.

To be safe I would stick a trouble light with a 60W light bulb in your bilge under the engine. That will provide enough heat to keep your engine safe down to fairly cold temperatures. This is what the local ski club does here to extend the season into October. Just make sure you position the light so the heat won't damage anything.

If you open the block drains and drain the exhaust, do you have any trapped water left in the engine? If not, I'd say go for it and drain it after you use it. No need to fog the cylinders or anything for a short layup. If there's access to the block under the thermostat, you might want to add a gallon of antifreeze after you drain it for cheap insurance. I'd trade a couple bucks for some antifreeze for run in the boat in March, but we've got a foot of snow outside and it's 20 degrees.

I've been under the impression that just one good freeze could do some major damage. But i dunno.