I have to suggest Tranquilo,
http://www.tranquiloaruba.com/
the boat is a 43ft monohull. I thing for what you are looking for it's perfect. Although I have never been on the boat or had the opportunity to meet Captain Tony, I am sure he could more easily accommodate your needs. Ask that he picks you up at the dock. It would be easy for him to sail the boat flat or with just bit of heal (tilt). It will be a more intimate experience then the Bigger cats.
The price may be a little more then the catamaran but is the moment and evening not the party that I expect is more important.
Cheers,
Mike