20 minutes drive in any direction from Kihei takes you well outside of Kihei, it's a pretty tiny place!
I think we get used to larger distances and still being inside the same city. The aquarium is in a town called Ma'alaea which is the start of the highway up to Lahaina and Ka'anapali.
http://www.mauioceancenter.com/
Not sure, but I think when you say Napili you'll actually be up in the Lahaina or Ka'anapali area, which means you're very close to Honolua Bay. It's considered one of the better spots for snorkeling so definitely do check that one out.
