Anytime you can take the time and go in a day early, is always a good idea. WE're also doing a southern cruise in Feb., but have early flight out, and also from the midwest, but I think we'll be OK. For us it just wasn't worth it to fly in late on Fri and get a hotel then, would of rather been able to get out earlier that day if I could get away. Personally, I'd just grab a cab to a hotel, we've stayed at the Wyndham in Old San Juan, its right across the street from the pier, but you will find most hotels in SJ are expensive, but do check on that one.
It sould be nice and warm even in Jan, the temps in the caribbean don't vary all that much and hopefully not much rain. It will sure beat a midwest winter by far. You may also want to think about getting travel insurance, its always a good idea.
Any more questions, just post them and someone will be able to help you!