I would be surprised if they would permit access via dinghy, but would be curious what you learn. We were there a month ago and made lunch reservations, but came in from Spanish Town - easy to grab a ball there, dinghy to marina and take a very short cab. Little Dix was very specific about the time for the reservation and we showed up 20 minutes early to grab a drink. After a little wait, they did let us in and sat us down early. Food and service was incredible and resort was spectacular. You could also purchase a day pass for the resort, much like what Oil Nut used to permit.