I shouldn't be. If your departure time is say, 5:00 pm, you'll need to be back on board by 4:30pm. I'd take the 3pm ferry back, just in case of traffic. They do have ferries that run to Charolette Amalie, just not as frequently as Red Hook. Charolette will be closer to the ships. Here
is a link to the ferry schedule.
Oh, if the weather isn't nice, don't spend your time or money going to Trunk Bay because you won't get to see the beauty. If it's a nice day, in 50' of water it looks like it's 3" because it's so clear. The beach is just like powder and there is shade available. Trunk Bay made the top 10 beaches of the world. If the weather's yucky, either Coki or Sapphire on St. Thomas would be less expensive and time consuming. Not as pretty, but sure beats any beach in the US.