We've visited Nassau twice, but not in January. The first visit we did the Snorkeling with the Stingrays shore excursion in the morning -- it was great! Then we walked into town to shop and visit the straw market. The second visit we took the ferry to Atlantis and hung out there for a few hours! That was great, too!
At CocaCay, I laid out on a great beach chair most of the day drinking Bahama Mamas, while my daughter snorkeled in the shallow water in front of me. My husband and son did a shore excursion, riding wave runners for a tour around the island. They were jazzed about that, and I got a great picture! Everyone that did that was happy! Then they went snorkeling out in the deep, deep water, and they loved that, too. The lunch on the island was like a picnic, nothing special but we were fed!
Happy cruising!