last fall it opened at the normal time - around 10:30. There can always be issues - especially with the first cruise out of that port - but this year the Wonder will be coming from Miami (I think) so there should not be any issues like with the trans Atlantic - where the Magic had not been into a USA port for several months.
There was lots of discussion last year if once you checked in if you could leave the ship to explore San Juan and return before All Aboard time (that was around 9pm). I don't know the answer to that. We arrived in San Juan several days before the cruise and had an awesome time. We boarded the ship around 6pm after we finished touring OSJ for the day.