A lot depends on when they dock/get off the boat. And return time. Kahului is on the east coast, all the diving is done off Kihei in South Maui or around Lahaina in West Maui. Due to the tradewinds most of the diveboats leave early - between 6-7AM for the run over to Molokini from Kihei or Lanai from Lahaina. Most also do afternoon shallower dives just off Maui in the afternoon. Probably going to necessitate renting a car for the drive over.
If they have time to be tourists, I'd say try to find something in Lahaina as there's more to see there after diving. But otoh the better dive operations IMO are in Kihei, B&B Scuba, Mike Severns or Ed Robinsons. Diveops leaving around Lahaina are Extended Horizons or Lahaina Divers. LD has the biggest boats so the smoothest ride over if anyone has seasick issues.
In Kauai, Nawiliwili is just a couple miles north of the Poipu Beach area, Sheraton Caverns is a good dive in that area. Again the issue will be that the morning boats leave around/before 8AM.
Contact SeaSports or Fathom Five in that area. I know FF runs a 1PM dive.
If they happen to overnight in Kona, the Manta Night dive is supposed to be spectacular. Jack's Diving Locker is one op that does it.
. B&B is usually back at the dock by 10am so you've nearly the entire day to enjoy some other Maui activities.
