to calculate (roughly) the amount of water you will need to displace to raise the boat, multiply the weight of the boat by 0.117. This will yield the number of fresh water gallons you need to replace with air.

In the case of my Hunter 23 (yes, I have a lead slug too) it weighs about 2400 lbs. If presented with a similar scenario, I would have to displace about 280 gallons of water in order to bring it to the surface.