I don't understand why you can't just use a 5/8" twist bit such as this one (scroll down and select "5/8"):
DEWALT Metal | Find Tools and Accessories
Forstner bits are for drilling flat bottom holes and auger bits are much less stable than a standard twist bit. Spade bit would work, but again, harder to control than a standard twist bit. I agree with drilling part way then finishing from the other side for a neat edge. And I can't see drilling the wrong size hole and then repairing it to get the right size hole when you can just drill the right size hole to start with? But, I'm not an expert in drilling holes in boats, just wood.