Welcome, Debbie! So glad you've posted, and especially happy for your upcoming cruise on the Diamond. I'm confident you'll love every minute! And as for your World Cruise....I'm already turning green with envy! ;)
I'm not so confident, however of my ability to locate some previous posts of mine

. Somewhere back in the cyberspace of LCT I posted about a luggage storage place in San Juan. It's called Audiogia or something like that. I promise to repost when I find the exact info. Anyway, it's a reputable and very reasonably priced company that will store your luggage for the day and then take you to the ship or airport, whatever transportation you need.
And another suggestion. If you're walking around Old San Juan and want to just mellow out for a while, may I suggest the Hotel El Convento. It's right in OSJ and has a lovely lobby/atrium/courtyard where you can relax and have a drink.
Have a wonderful cruise, and again, welcome! :lct: