Middle fore to aft and as low as you can get top to bottom. All of the passenger/cabin decks are above the true middle, top to bottom. But the lowest passenger decks probably do not have balconies, so go as low as do, and midship.
For the itineraries, you will need to go to each prospective cruise on the cruiseline's website OR possibly a larger agency's website.
I have not yet cruised the Caribbean, so I will let others suggest. We are booked on a Holland America cruise to the ABC Islands (Aruba, Bonaire, Curacao); that is Southern Caribbean. At one time we booked Royal Caribbean Cruise Line from San Juan to the ABC's, but had to cancel.
Good luck deciding and Welcome to Cruise Critic!