First boat leaves at 9:00 am... last boat leaves the island at 5 pm. The boat comes as needed, just wait on the pier and they will come get you. Never more than a 5 minute wait. The only difference is when there is a wedding planned, I think it closes at 3pm, but it will open 1 hour earlier the next day and and stay open 1 hour later. (someone correct me if this is not so) I hope you both enjoy the island. I'm sure you will.
The boat normally heads over beginning at 9 am and all day long until it closes at 5 pm. If there is a wedding or special event scheduled that day it may head over at 8 am and close at 4. They normally advise ahead of time for the earlier schedule!
Times can vary. Usually from about 10 AM until 4 PM, maybe 5. It's been a couple of years since we were there. The hours are shortened on days when there is a wedding booked on the island. They will close the island early for a wedding. However, they will usually add time the day before or the day after a wedding on the island to compensate. The island may also be closed on Red Flag days. Red Flag days are when the weather causes the waves in the ocean to reach a certain height. I think this is when the waves are over 3 feet, but don't quote me on this. This is done for safety.