From the point of view of undue excitement, pouring molten lead into cold water produces some steam and lead shot and very little drama. The hot lead sinks below the surface and the steam given off condenses before it gets to the surface and the rest of the water just warms up a bit.
Pouring molten lead into an enclosed space with any damp involved is a very different matter, since the very small quantity of water is suddenly turned into very hot steam and tries its best to take up about 1600 times its original volume causing a fountain of steam and whatever was there, mostly lead that would still be molten. This applies to a few drops of condensation in the bottom of a hull and any residual damp in a plaster mould. When trying this sort of thing, I have always cooked the mould for a considerable time in a low oven to make sure that there is no moisture there - as a bonus, if the mould is pre-heated, it is less likely to suffer thermal shock and crack when the lead is poured.
However dry I got the inside of the hull, I would still be very uncertain about pouring anything as hot as molten lead into an expensive hull, no matter how certain anybody else was about the water on the outside being able to keep the hull from overheating to the point of damage. Fibreglass is a pretty good insulator, and the weighing merits or otherwise of a thin or thick hull to either withstand damage versus conducting heat away into the water is a journey into the unknown that I would not care to take with something that I had paid for.