A forstner bit is the wrong tool, good for flat bottomed holes, not great for this. Like others have said, a small pilot hole with a conventional twist bit works best, then open it up.

If you really want a tool, you can make a tapered piece like a wedge, so that you can have a flat surface to drill on, put a couple sheet metal screws through it so the points just barely protrude through the back side, then the "tool" won't move around against the boat when it's pressed against its side. It's a very basic drill guide, but it works.

Bill H.