You could get that harness, or you could spend about a fifth the money and get a regular rock climbing harness at REI or EMS or something like that... The Spinlock Mast Pro harness is made by Petzl, who makes regular rock climbing gear. You can buy a regular rock climbing harness that does pretty much everything the MastPro does for a lot less money.
I wouldn't recommend using the Mast-Pro or any mast-climbing harness for a sailing safety harness for one major reason. Climbing harnesses attach fairly low on the body, below the center of gravity, unless you get one of the full-body harnesses. That means that you're likely to get dragged in the water face down by it...since it is below the center of gravity on your body...
Safety harnesses for sailing tethers are upper body harnesses and need to go around the upper torso above the sternum. This is why many of the PFDs with integrated harnesses, like the Mustangs and their WM cousins are not for people shorter than 5' 8". While their literature doesn't say this, the fine print on the PFDs instructions CLEARLY DO.
If the harness extends below the base of the sternum, the harness is on the floating ribs, rather than the ones attached to the sternum, and if you fall overboard-the forces transmitted by you falling from the jackline to the tether to the harness can break the floating ribs and cause serious internal bleeding that can possibly kill you.
Just some food for thought....
BTW, if you want a PFD with a good safety harness and are relatively short, the Spinlock Deckware Pro and Deckvest seem to be the best choices... I use a Deckware Pro for that reason. However, neither is USCG approved.