Maui in Dec
I used to live on Maui and the northwest part of the isle gets some crazy surf in the winter. The area north of Lahaina gets some wicked waves and that's the last place you'll want to go snorkeling. Stick with Wailea since the beaches will be calmer.
For most beaches on Maui, they'll get windier as the day progresses. This can make any calm beach choppier by mid-day making snorkeling a little more challenging. The tradewinds come from the northeast part of the island towards the southwest, so you get more coverage from the wind if you stay towards the south (the leeward side).
The drive from Wailea to Lahaina can be a stretch, like 45 min or something. Lots of traffic because some drivers stop on the beach road like nobody is behind them so they can look at the breeching whales. They are great to admire, just watch the road!!! However, if you are from any metropolitan area, you know how people drive.
If you are looking for some good local food, head to Cupie's Drive Inn on 134 Kamehameha Ave, in Kahului. If you are up for a good breakfast, Charley's in Paia or The Gazebo out in Napili serve 'em up good. The Gazebo has a nice view of the humpback whales in the winter and they have some great mac nut pancakes. Oh I miss home....