Essential and Optional Gear for Fishing
Pliers - Don't leave home without 'em!
Gloves - Fishing gloves such as those made by AFTCO, Pelagic and others are important to protect your hands, especially at the end game when billing a marlin. (Confession: I forgot mine for two of three fish and did OK, but that's not recommended.)
Terminal tackle - You don't need much and most of that (circle hooks and a few light weights) are available from the mothership. I'd recommend bringing along a few 100- to 130-pound wind-ons. Optional tackle might include some 1/4- to 1-ounce rubbercore sinkers, metal jigs, spoons and big topwater plugs. If schools of surface-feeding predators are cruising through, you could opt to carry a second rod rigged with a plug or jig to toss to them.
Sea anchor - Not essential, but from my experience, a very small sea anchor (that you can easily toss over with one hand during a long battle) is well worth bringing (see "Fish-Fighting Tips" for more explanation).
Depth sounder - Absolutely unnecessary. However, I admit to bringing the Humminbird 797 I use on my own kayak at home with a suction-cup transducer mount and powered by Daiwa's unique and very compact Dendoh deep-drop battery. A sounder can be useful for seeing baitballs or fish deep enough to avoid visual detection. Also the GPS /plotter in a combo unit provides a handy way to keep track of where you are at any time.
Mini-EPIRB - If you have one, by all means, pack it. Odds are scant you'll need it, but it never hurts to know it's there.
Camera - The smart money says to bring one, either waterproof or in a waterproof case. Small is best. You're likely to get some incredible photo ops, particularly given your sea-level perspective.
Essential and Optional Gear for Safety
Whistle and line cutter - While Marlin Masters provides most safety gear, I recommend you bring the world's loudest (and completely waterproof) whistle, the Storm Whistle (www.stormwhistles.com). Line cutters are part of your gear, but if you want to bring along a backup, it's cheap insurance. Keep both handy on a lanyard around your neck.
Fluorescent flag on a pole - Marlin Masters should provide these, but do ensure the flag is hoisted and well above your head before you leave the mothership. It really makes you significantly more visible in the swells to other boats. Highly recommended!
Foul-weather gear and layers - On calm days, even into January, you may enjoy short-sleeve weather, but be ready for misty, cool Pacific weather as well.
GPS - Marlin Masters provides waterproof VHF radios; if you have a handheld GPS, bring it, in the event you want to give someone a precise fix at any time.
Water, drinks, food - Take plenty. Most of the time, the tender can and will run supplies to you, but if you bring them along, you won't have to rely on that service. More than once I found myself popping a top in the middle of a long marlin fight, glad I had brought food and drinks along when I left the mothership.