We are heading down to KW at the end of May on Harleys and want to rent a boat for a day while we are there. Have seen several places to rent a nice CC from. We plan to do a little fishing, some snorkeling, sight seeing and maybe find a spot to just beach up with some other boaters and do a little partying. My questions is where to go. Would it be better to just stay around KW or I found a good deal in Marathon also. We don't mind jumping on the bikes and heading back north, we would just like to know where you all think the best place to boat for the day would be.

At mm28.5 there is a place called Dolphin Marina (http://www.dolphinmarina.net/index.html) that rents 20-22' CC. I have rented from there a few times and every time things worked out well. If you want to go snorkeling, you can go straight out from there and get to Looe Key. I believe its about 5-6 miles out. Easily found with a gps.
I was talking to a local there last time i was there and he was telling me about a local sandbar that they go to. It was close to the boat rental place but i don't exactly remember where he said.

I'll check it out, thanks for the info.

You definitely need to check out Bahia Honda. There's always some raft-ups on the Northeast side of the island. Facilities on the island too.