Kevin,
Because the packages vary so much, they don't just post prices on the website. What you need to do is call them and ask about what is available, and when. There is a
website page, but it doesn't really share much information. So definitely call 407-WDW-PLAY (407-939-7529).
Some of the choices include the Pirates and Pals Cruise, which I really enjoyed. But it is not dinner, and it is not particularly romantic. It is fun, however. There are other fireworks cruises, both for the Magic Kingdom and Epcot, but again they are not dinner. There are also some kids camp-type cruises, but again not what you are asking for here.
The one, however, that does match your description, is the rental of the Grand 1 yacht, which is on the Seven Seas Lagoon by the Magic Kingdom. Costs are based on a number of factors, but know you are going to have to drop several hundred dollars for the privilege. Call that number and they can tell you more!
Have a magical vacation!