Never made reservations(based on advice of many). Taxi from airport and purchase 1 way ticket on "Next ferry leaving".

Returning from Tortola, I had purchased a "round trip ticket" in STT, but all that does is allows you to stand in line, give that "round trip ticket" and then pay for bags... so no time savings there.. And if that ferry unavailable (full or canceled), then you have a paid ticket that you cannot use... Not worth the $5 or so that a Roundtrip ticket costs.