If you leave it at Sosua beach (casino side) at entry, the guy watch it for you, but then he wants 50/100 pesos. You can leave it a bit further up the road for free, but no one will watch it for you there. I usually leave it there for free with a strong chain lock and hope for the best - so far so good, but who knows...
I think you should have a lock of some sort, without it they can hot-wire start a bike here in 30 second. They can also steal the battery, I have seen many people complain about that. Oh, at night leave it somewhere you can see it, if not it might not be there when you come back.
I guess if you are renting, clear all this with the renter, if he has insurance for theft, battery and damage etc....