I would not rent a car. You are only there for 1 day.....I would take the ferry over from Marigot...

Of course you should rent a car. There is limited to non-existent public transportation especially to all the beaches overseas discount airline tickets you will want to visit. I guess you could take a taxi since there are 3 of you - but your own wheels will give you the flexibility of heading any where you wish and stopping whenever you want to check out a gorgeous view (which you will find at every turn).
There are 30+ beaches so don't try to do too many in a day - although a visit to world famous Shoal Bay is on the top of everybody's list and should also be on yours, but after that don't just head for the touristy spots - instead, just simply choose a random beach which you spot on the map and check it out - you might just have it all to yourselves.
I would not rent a car. You are only there for 1 day.....I would take the ferry over from Marigot....once their, have a taxi take you to Elodias on Shoal Bay East...hang out there for a while...then, if you are facing the ocean, head right down the beach, round the curve and head to Gwen's for lunch......and there is a massage hut right before it......just my opinion...vs having to be on a time schedule so you can hit various beaches. Just sayin!
ps....chances are you'll run in to a guy named Nell on the Ferry...he was great....he took us to Elodias and then picked us up in time for the last ferry back to SXM.....he'll tell you times, etc...no stress...just one of the most beautiful islands you'll ever see!