The whale season is typically early Dec. to Late May. By law you can't approach the whales closer than 100 yds. while on a boat so a liveaboard is redundant since the dayboats get you out to the same places. There's only one liveaboard in Hawaii anyway - the Kona Aggressor. It travels along the coastline and is sometimes called the cellphone liveaboard since yours will work most of the way except maybe the far south end of the island. https://www.aggressor.com/kona.php
I'm not positive but I don't even think you're allowed to approach them underwater - they have to initiate the contact.
Here's some Maui info: FAQs | Save Our Oceans I'm sure the Kona whalewatch tours have similar websites.
You'll get whales on the dayboats in whale season--but usually only whalesong. Underwater sightings are pretty rare--libeaboard or dayboat. In the summer I often see Tiger sharks from the boat (although only once while diving). The caveat: a liveboard (Kona Aggressor) is really the only easy way to get in more than 2 boat dives every day in Kona..although you could shoredive and get 4 (if you really push). Also, the Aggressor (while staying close to the coast) goes from North Kohala all the way down to SouthPoint (Rainbow Reef)--so it definitely covers dive sites you would not see from a day boat.