We had the bilge plug fall out of my first boat, a 15'9" runabout with a 115 Merc. We were on the Willamette river in Portland, and using Ross Island as a place to have everyone lounge and do standing starts waterskiing. The bilge plug on this particular boat was put in from the exterior of the bilge, not from inside.
We had a plastic bag from somewhere, and found a stick about the right size to plug the hole. I used the same suction method as above to get the bilge emptied, then we jammed the stick with the plastic bag in the bilge plug hole. We were able to get everyone back to the dock and the boat on the trailer with this little bit of redneck engineering!
Needless to say, from that point forward, I always had a spare bilge plug in the boat, just in case!