Annoddddd said:
We never make reservations.
After landing hop over to the ferry terminal and just catch the next ferry out. You never know what's actually running or not beforehand.
Speedy's has a pretty defined schedule on Tues-Thur-Sat for the VG - St. Thomas runs and follows it pretty closely. With that said, we've never had to make reservations ahead of time.
The OP didn't say when they were traveling, but Speedy's won't be doing the Thur. runs from Sept 3 - Nov 26.