Rent a 4 wd vehicle is the only way to go

Pnut- first of all, rent a car. Taking a taxi doesn't work well for STJ and you will wish you had a car to do all the exploring you'll decide to do spontaneously.
Secondly, there are about a million posts on this board for boat trips so set aside a couple of hours and search them. We took the Limnos charter one last Nov to the Baths followed by snorkling at Norman Island. The differences between the different day tours are: a) how big the boat is- Limnos was NOT crowded so we had lots of room to breathe while other smaller boats had people literally jammed in like sardines b) what the focus is- they all have free flowing liquor which is covered by your ticket but some boats specialize in just going to the different bars while others take you to snorkling places. IMHO, you can drink anywhere, we wanted a trip that would take us to see something other than a plastic drink glass full of rum and thought LImnos did a great job of that- we would not have wanted to rent a boat I don['t think to go all the way to the Baths by ourselves.
But of course, the vacation is yours, just check out all the opinions on board here for the various charters.