I would suspect that it's a bad ground on the gauge somewhere... It's the simplest gauge, it just reads the hot vs ground, that's it. If it went to zero and the boat still ran, it's not the battery. If your alternator (or regulator) were bad, you would slowly see the volt gauge go down to zero. When it gets below about 5v, you would notice that it's not running properly, especially at idle.
Did you try to start it again when you were back at the ramp? That's a good way to tell - if there's enough juice to crank it over, you know it's either a gauge ground or the gauge itself.