Puamana is one of the nicest condos in Lahaina. Lush green grounds. Gated community at the south end of Front Street. A fairly long walk to the main section of town, but reasonable by bicycle. The water in front is well protected by the reef in front, but the reef itself is mostly wave-beaten and broken coral. It's a pleasant walk along the oceanfront on up to the restaurants at 505 Front Street. I don't really think that the area in front of Puamana is worth diving, particularly since until you get outside the reef a couple hundred yards offshore, the water is mostly just waist deep. It is worth exploring with mask and snorkel to confirm that, though.
Olowalu, better known as MileMarker 14 is a nice snorkeling spot a few miles to the south, and is a reasonably good dive.
Just to the north of Lahaina are several popular scuba spots --- Post Office, Canoe Beach, Black Rock, Old Airport, and more. Pacific Dive and Lahaina Divers, both on Dickenson St. in downtown Lahaina rent tanks, as do the many stores of the Maui Dive Shop. The Maui Dive Shop stores all have a free magazine that includes basic info on shore dives throughout the island.
You picked a really good time to come to Maui. The Christmas crowd (including my two daughters, a son-in-law, and 3 grandchildren) will all just be leaving the island about that time. There should be lots of space on the boats.
I recommend that you PM, Kidspot. He's a Lahaina-based SB member that does shore dives with many visitors. I'll be in Kihei during that time, and after hosting visitors for a couple weeks will be itching to go diving. PM me if you want to get together for a dive at one of the S. Maui shore spots, like Ulua and Makena.
Charlie Allen