My Draco 2500 twincab has been repowered with VW td engines and tunnel
drives of my own design, for which I have added a 1 ft long box to the
stern. In fact it is an extension of the hull that supports the 2 tunnels,
rudders etc.
Although I've spent many hours on my back under the hull laminating and
painting, now that the boat is back in the water, a steady trickle of water
is entering the bilge through one of the holes where the stern tube is
bolted to the stern. The bolts are not visible outside, they are in a grp
walled cavity filled with polyurethane foam.
I would have bet anything that the construction is watertight, but I'm glad
I didn't.

My plan is to locate the hole by pressing a sponge soaked in dye against the
suspected area, with a helper inside who looks for a color change of the
incoming water.

Is anyone familiar with a product or procedure that can successfully seal a
small hole in a submerged object?