You'll find that the rental price is the same, regardless of the # of people.
I know that when you make the reservation you can make special requests (at a price of course) for champagne, food, etc. to be on the boat. Supposedly they will not allow you to bring your own food or beverage onboard.
If it is just the two of you, you might enjoy the Breathless more than one of the pontoon boats. But I do not know if the Breathless will allow champagne and/or food. So definitely ask when you call to make the ressie.
Also, definitely plan this ahead of time, and plan on calling the full 90 days in advance to schedule the cruise with Disney.
HAVE FUN!!