Grounding the generator certainly would be the prudent thing to do, and wiring it to the keel would be effective. My catamaran doesn't have a keel, but it does have a grounding plate for the dc.
When I first started cruising, I didn't know where the ground was on my boat, so I just attached a length of wire to the generator ground and tossed it overboard. A little rednecky, but effective. Someone mentioned that you could electrocute nearby swimmers that way, but the same is true for any ground on a boat. Besides, I think it's much more likely that electricity would slowly dissipate through the wire, instead of building up on the generator.
As I continued cruising, I observed that no one I met bothered to ground their generator. And I too got lazy, and stopped grounding my generator. You should however be concerned about carbon monoxide. Make sure the generator exhausts
downwind, and that its exhaust
is not getting caught in an eddy created by a bimini, etc.
Also, I don't know why you need so much electricity for your event, but even the quietest generator is going to effect the ambiance of being on a boat out on the water. If you have to have a lot of juice for your event, maybe it would be best to stay at the marina and connected to shore power for the event.
Gemini Catamaran Split Decision