Several things to consider beyond what boat to book:

If you're bringing small children, don't book a Zodiac tour.

If you have any issues with sea sickness, morning will 9/10 times offer the flattest water. Also makes spotting easier.

To add on to the sea sick point, if you're not a regular on the water, take some kind of motion sickness medicine as directed. Bonine IMHO is the best conventional remedy, but there are many alternatives.

Check out the boats you consider rather than calling blindly or relying on pictures from the internet. The last thing you want to happen is to pay for a trip and upon arrival at the docks, you realize that the craft you're about to get on is less seaworthy than the SS Minnow. Chances are you'll be fine, but there are a few less reputable operations out there.

Call the Kona Charter Desk at Honokohau Harbor and ask some questions. There is a wealth of information to be had from the people who work there.

There are several research groups on the net who do not give tours, but do report their findings pertaining to frequency of sightings, location, etc. A valuable resource and will help to avoid disappointment if the numbers aren't there.

If you're staying close to town, Kailua Bay has been known to at times have a decent number of mothers with calves. the shallower water lends to protection as the newborns get their bearings, hone there swimming skills. This can lead to amazing free encounters.