Murphy's Law. The one time your camera is turned to the wrong settings, that will be the time a 700 pound marlin goes bananas right next to the boat. So, always be ready!
I've been on media boats with pros who've lost the shot because they got complacent – changed their settings so they could get a portrait on the boat because there was no action, and then forgot to change the settings back. Ninety-nine per cent of the time this won't matter, but you'll never forget that one per cent when you missed the shot of a lifetime.
Buy the best lenses you can afford. Camera bodies change every year, but lenses last forever. The best body bargain is to buy last year's model, in other words, get a 50D just after the 60D comes out. Amazon.com will give you the biggest selection and prices.
To take action shots, I suggest setting your camera's program to Aperture Priority (which lets the camera set the shutter speed) and F-8 at 400 ISO. If it's early in the morning or cloudy, I raise the ISO or adjust the f-stop and take a few practice shots to see how they look.
Great thing about digital is that you can instantly see what you're shooting. Remember to check your settings every time you pick up the camera. Conditions change and you don't want to be shooting 4000 ISO in bright sun.
How to compose a photo is an important skill in photography. So, always think about what you want to have in the photo.
Close -up or wide angle? Horizontal or vertical? Are the eyes in focus? Do I need a flash to bring out colors on a cloudy day or do I just want to fill in the shadows on faces? Set up the shot by moving people around or changing the way the fish is being held. Use your imagination!
It's easier to get a shot of a jumping marlin from deck level. Once hooked-up, clear the cockpit of stray rods that might get in the shot. I always have a towel over my shoulder to protect my camera from splashes when backing-up.
For the best marlin jump shots, practice following the line where it cuts into the water. Be ready as it starts to rise quickly, as this means it's about to jump. Set the camera to continuous shooting, and keep your finger on the shutter as it leaps to capture the entire procession of jumps. Be careful not to zoom in too close as the fish can easily jump out of frame. You can crop-in on the action when you fine-tune the shot later.
Modern digital SLR cameras, with fast focus and high frame rates, make taking photos of jumping fish almost as easy as point-and-shoot. On my Canon cameras I've discovered a little trick that makes getting jump shots even easier, without needing to press on the shutter to initiate focus.
On the back of my Canon 5D and 50D, there's a star button (*) that focuses the camera when you press it with your thumb. I use this button to constantly focus on the water's surface at the distance I expect the fish to jump. That way, when the perfect shot appears in your viewfinder, you simply press the shutter and the camera is already in focus – saving focus time and enabling you to get exactly the photo you want, or more frames in a motor-drive sequence.