We've been to St. John the last week of January in both 2010 and 2012. The weather was great both times. It rained for a few hours two days, and some nights, but overall it was beautiful -- high in the mid-80s and sunny.

BTW, I've also been to Maui 4 times, and I've never heard that snorkeling is bad in January. Certainly, the northern ends of the islands can be too rough in the winter, but I don't think you'll have trouble finding a good spot to snorkel if you choose the protected coves.