Hi Stacey! Congrats on your upcoming vacation and thanks for your question.
While you can certainly take a boat to Downtown Disney to hop a bus over to Grand Floridian, I believe the quickest route would be to take the resort bus to the Magic Kingdom and then either take the monorail or boat launch to the Grand Floridian.
The bus ride from Port Orleans to the Magic Kingdom should take about 15 minutes give or take. Upon arrival to the Magic Kingdom, you can either choose to board the resort monorail or take the resort boat launch. Grand Floridian will be the 4th stop on the Monorail loop from the Magic Kingdom and takes about 10-12 minutes to arrive this way. Grand Floridian will be the first stop for the boat launch and this relaxing ride also takes about 10-12 minutes. Please keep in mind, the boats only arrive every 15 minutes or so, so depending on your timing, you could be waiting longer for a boat to arrive, whereas monorails arrive more consistently.
In general, the monorail is probably the quickest bet, unless you are dining at
Narcoossee's, one of my favorite restaurants, which is located steps away from the boat dock... otherwise, you will have to walk across the resort to reach the main building and other restaurants after getting off of the boat.
If time is not of the essence and you have a moment to spare, the scenic boat ride is so very relaxing and offers unique views of the Seven Seas Lagoon, The Grand Floridian, and the Polynesian Resort.
Hope I have answered your question and please let me know if I can be of further assistance! -Amanda