Going a day early is always a good idea! Saves you from worry on what could go wrong with your flight and you are much more relaxed getting on the ship. You will love the Destiny! On deck 8 I believe or it could be deck 7 at the very front of the hallway there is all cabins but if you look to your right you will see a door that goes out on the very front of the ship. Not many people find it and it is a great spot! The way they have the railing it blocks the wind too. Also outside the centrum they have deck chairs with padding that is quieter than the pool areas if you are so inclined. This is a great ship - we did a lot of exploring on her. I've heard many people just taking cabs in San Juan - I actually flew cruise air there because it is so high from Texas to fly there. Actually post the question as "taxis in San Juan" and you may get more responses.
You'll have a great time! Also the ship is Huge! If you can please get a central cabin or you will be walking a whole whole lot! We like to go back and forth to the cabin from the pool and it took forever on this ship with a back cabin. Debbie