I have cruised Disney several times in December and January. It's nice to leave the snow at home and whatever weather we have seems wonderful by comparison. There are websites where you can look up average temps and average ocean water temps. December should be pleasant, warm enough for water and beach activities. I do pack a lightweight shortie wetsuit for snorkeling but only had to wear it once when a breezy wind made it feel a little cool when getting out of the water at
Castaway Cay. Since it gets dark early at that time of year, prepare for breezy and cooler conditions on deck after 5 or 6 pm in the evenings. My wife would warn you that the temperatures inside the ship can be over air-conditioned and a little cool. She brings some light sweaters and can often be seen with a blanket in the theater. I would say to plan for warm days on the beach, but pack an extra layer just in case.