Hi, I am from PR. and must say that the info here already posted is spot on. Puerto Del Rey in Fajardo is your best/saffest bet. their are a whole bunch of sailboats on the hard
with people doing exactly what you want to do. They tie up the boats really well and are reasponsible people. you could also leave it on a dock
at this Marina but I dont know about finding somebdoy to keep a constant watch on the boat.
In Culebra and Vieques their are no real marinas. you could leave your boat in Ensenada Honda
in Culebra where their are various boats their long term but you would have to make some kind of arrangement with a local to keep an eye on your boat. If I were you I would go with Puerto Del Rey or the place mentioned in St. Thomas (I have not been to this area of St. Thomas.)
If you end up leaving the boat in PR and would like somebody to look after it once in a while let me know. I go sailing out of Puerto Del Rey about 2 a month and would have no problem doing this for you for free.
Good Luck! Daniel