Some have special offers for cruise ship divers that include picking you up at the port. If the dive operator you're considering is contracting with your ship then they probably won't let you book separate from the ship's excursion. Check your ship's sail date "Roll Call" on Cruise Critic and see if there are any other divers setting up diving on their own. If you can gather a group of four or more divers the dive shops may be willing to set up a special excursion for your group outside their normal dive times.
Email the prospective dive operators and ask them if they have any openings for the day you'll be in their port. Be sure to check your ship itinerary carefully so that you inquire about the right date and time. One challenge you may run into is the difference between "ship's time" and local time. Most (but not all) cruise ships like to stay on the time zone of their departure port. Miami is on Eastern time zone but Cozumel is on Central time. This gets even trickier with Daylight Savings time. Not all islands observe Daylight Savings time so be sure to search the Internet for local times at each port, then compare that with the time at your departure port. Some dive operators know when each ship will arrive and will work around your schedule, but most will expect you to show up in time to catch their dive boat.
Ask if they provide transportation from the port, and if not, what the cost of a taxi is to their shop. Ask how long it should take to get there and if they have lockers for any gear you leave at the shop. If they do have lockers be sure to pack a padlock. If not don't leave anything behind that you can't afford to lose (i.e. like your ship card or passport). If you are going to rent gear tell them what you need and ask what that will cost.
Each shop will email back with an affirmation or denial. Most will ask for a deposit and should give instructions for paying that deposit. Don't email credit card info, instead ask for their fax number and send it via fax. Some will take Paypal. Be aware that these deposits are usually non-refundable if you cancel less than 48-hours before the scheduled dive. Once you have a confirmed reservation (that acknowledges your deposit) print that confirmation and bring it with you on the day you arrive at that port. Mistakes can be made and they may overbook their boat. If so you want proof that you have a reservation. While this is rare it has happened.
It is wise to set up your diving as soon as you have booked your cruise, but at least 2 months ahead of time. Good operators fill up quickly. A few weeks before the cruise email each dive op and remind them when you will be there. Email them again once you are on the ship, especially if you are expecting someone to pick you up at the dock. Confirm where you will meet them once you disembark. Cruise ports are unlikely to let the dive op drive right up to the main gate because taxis and tours are given preference. Make sure they are aware of the time you must be back at the ship. Always plan to return at least an hour prior to the last ship tender, or if the ship is docked, an hour before the ship sails. Traffic can get very congested near the port so plan accordingly. The last thing you want is to be running for the ship with all your gear.
One last thing to remember, you are on their time schedule, commonly known as "Island Time." That means they may be there when you get off the ship, or they may be 30 or 40 minutes late. You are on vacation so don't get all worked up about it. One way to avoid stressing over this is not to plan anything else in port that day (i.e. shopping, drinking, site-seeing). We don't feel comfortable booking diving in a port if the ship will be there less than six hours, unless the dive op, and the dive sites are very close. You'll find that a little pre-planning makes the adventure so much more enjoyable. Here's to cruise ship diving, bottoms up!
