I had to buy a sweatshirt on my cruise last week. I should have packed one, but forgot. Layers are the way to go. You'll probably hit warm weather by the second or third day. As soon as you do, they'll crank the air conditioning up full blast on board. I always take a sweater to wear in the public areas on the ship. You will have a thermostat in your cabin, so you can regulate the temperature there. You'll probably have to adjust it again after the cabin steward leaves, they seem to love to keep the cabins icy.
Have a marvelous cruise!