"The one from Red Hook is faster, cheaper,"

I'm sure Alana or STT Res. will comment, but there is a difference of distance and cost of taxis to cover that difference in distance...

I think to save the cost of even tipping a taxi driver I would try to schedule around the CA ferries.

Oh edit: you didn't mention how many people would be going. This could make a big difference.

Exactly. It's six of one and half a dozen of the other where time and cost is concerned. For the max time on STJ the Red Hook ferry will work out best for you. Leave early AM and you'll have a quick ride to Red Hook as the commuter traffic comes into town and not out.

Thanks. 4 adults, if that matters.

Red Hook will be the most flexible given what you are trying to do on STJ. If time is more important than money...
Not sure if any of your party is prone to motion sickness but the CA ferry tends to be a rougher ride on open water and given the longer time on water that combination might set the STJ mood back a bit.

Chris at Work wrote: ... If time is more important than money...

The cost comes out the same whichever ferry you take when your starting point is town.

Water Island to/from STJ: I try to "mix it up". I take the 8:15 into the city (sorry....couldn't resist breaking into song!), take dollar taxi/safari to the Tent Market, cross the street and take the Charlotte Amalie ferry to Cruz Bay (~40 mins). I buy a one way ticket because after I complete my adventure on STJ, I want some flexibility on when to return to STT. There is more than one ferry company and if you purchase a RT ticket from Company "A", my understanding is then you are "locked in" and will need to ride with Company "A" (and their schedule). In the end, buying a one way ticket might be a buck or two more but I like the flexibility to take whatever ferry is leaving Cruz Bay on the return trip at the time I am ready to leave.

On the return trip, I like to take the Cruz Bay to Red Hook ferry (~20 mins). I like to explore East End STT before having to head back to Crown Bay Marina/Water Island. In Red Hook, you might have to walk down a bit to catch a dollar taxi/safari and you will have to xfer at the Big Kmart to catch a dollar taxi/safari that will ultimately allow you to reach your Crown Bay Marina destination for return to Water Island. {Edited: You of course can also catch a "regular" taxi. It will cost a bit more but if you're running short on time, for sure you will want to catch a regular taxi.}

Obviously, plan your day/time to ensure you don't miss the "last run" from STT to WI whether that's the 6:00 or if you take a late run (inform the Captain in advance) 9:00.

The first ferry out of CA isn't until 10AM and the OP wants to enjoy a day of beaching and barhopping on STJ so taking that option leaves precious little time.

Catch a safari van to Red Hook and back. What is it, $2. However, they don't really have schedules and you have to be on highway as they rarely go by Crown Bay Marina.

With the passenger ferries, it doesn't matter which franchise you buy your ticket from as they will both honor it.

Not so for the car barges.
You must come back with whichever you went over on or purchase a new fare for the return.

By the time the CA ferry gets you to STJ, you've wasted 1/2 the day.
Any way you look at it you're going to spend money.

Rent a car, drive to Red Hook, pay for the RT on the barge, drive yourselves around STJ on your own time and schedule, come back to STT, explore Red Hook, have some dinner and arrange for the 9 PM ferry back to WI, return car the next morning.

That's actually your best and cheapest option and has your on your schedule, not waiting for taxis to fill up in STJ before they'll take you anywhere while charging an arm and a leg for it.