One hour prior to early entry.
The boat is much more relaxing and is almost exactly an 8-minute ride, so much nicer than the bus, although the bus ride is about 3 minutes quicker.
The boat arrives with its own security check and docks much closer to the tapstiles than the buses. I have found any time saved by the quicker bus ride is lost early in the morning with the walk from the bus depot to what can be a very long line going through the metal detectors.
There are two boats and they arrive every 15-20 minutes... just like the buses.