What are you cutting the hole in? If it's just fiberglass, or thin plastic with vinyl over it, then you can use an air saw like this:
That saw will zip it right out.
A dremel won't work as well if there's any kind of thickness to it.
If it's a panel that's got vinyl/foam/3/4" wood, then fiberglass, like most powerboats do, then you'll need a jigsaw with at least a 5" wood blade, similar to what chpthril said.