Suggestion: Dry it off with a towel, and then immediately hold a paper towel against the surfaces around the stern tube. See where the towel gets wet first. Likely bets are the bellows to stern tube clamping and (less likely) a separation between the stern tube and the hull
. Remember that water
can flow along the bottom of a tube, so if you find water
on the underside, then trace it uphill the same way.
If it's a crack between the stern tube and the hull
, you'll have to pull the boat
, dry it out, and start your repair from the outside. Let's hope that it's not that.