I tried it with the plunger and a bucket and I couldn't get a good enough seal on the plunger for the boat to actually suck up through the plunger. Most times people use plungers they are hooked up to a pressurized water supply, not a bucket, so the seal isn't as critical.
What I suggest doing is putting the bucket of antifreeze in the boat and connecting a hose from the bucket directly to your raw water intake one way or another. In the past I have just pulled the raw water hose off the through hull fitting and stuck that big hose directly into the bucket of antifreeze. That works like a charm and it is only 2 hose clamps that need to be removed.