st. martin to st. thomas

We will taking our honeymoon cruise on the Valor from May 22-29. We looked at possibly going over to St. Thomas while at St. Martin. Has anyone done this and what is the best way? I'm know a ferry but how do I book it and how much does it cost.? or will this take up too much time? thanks

sorry about that. I ment to say from st. thomas to st. john