Hi ChadS, welcome to Cruise-Chat. How long are you going to be in Key West. Normally, they have 1/2 day charters. I googled Key West fishing charters and got 302K hits. Make sure when you're arranging one to let them know you're on a cruise ship and what time you need to be back to the ship. For example, if you're departing Key West at 5pm, you'll have to be back on board no later than 4:30pm.
The last fishing expedition I went on down there was on a land vacation. We went out on a drift fishing boat and it was an all day excursion.