We took the sunrise 6:30am boat from Lahaina harbor and so glad we did! The boat was the Ocean Quest which was one of their two level boats, that supposedly holds about 110 people. There were only 40 people on our boat though, probably less crowded due to time of day. It was AMAZING. The sunrise over the harbor is just gorgeous. We saw lots of whales... no breaches, but lots of dives, tails, spouting, etc. One even came pretty close to the boat. The naturalist guides were wonderful as well.
We are also all prone to sea sickness, but none of us felt sick. The only time you really feel the boat rocking is when they stop to listen to the whale songs. When you are moving it is better. And the Ocean Quest is one of the large boats in their whale watching fleet.
Definitely book online - some of the tours sell out, esp the 9am ones, and you probably want the harbor that's most convenient for you.
Parking for Lahaina is easy for the early cruise - there is a free parking lot at Prison and Front street, free for 3 hours, and a 3-4 minute walk to the harbor.
There is supposedly coffee and snacks available for purchase on the boat. Many people brought Starbucks or their own food.
A+++ all around!