Gear
If you are serious about getting good photographs, a 35mm SLR system with a medium range zoom is still the best option. You just need to care for it. For less serious photographers, a waterproof “point-and-shoot” camera is probably best. These can be whipped out of a handy pocket without any worries about rain or splashes.

Composition
Basic principles of photography suggest that by simply deciding what you want before you squeeze the shutter you will get better results. This is undeniably true. Here's some tips to help you think about composition.

The basic “rule of thirds” goes a long way to avoiding just another humdrum photograph. Split the viewfinder horizontally and vertically into thirds and place the subject or the center of interest at the junction of these lines.

Use the space created to tell the rest of the story. If someone is moving across your plane of view towards a point of interest, place the subject in a corner and leave the space in front of them to tell the story. If they are paddling or moving away from something (a large shark!) tell this story by making sure it is behind them.

Once you've decided what's in your shot, don't look at the kayaker—look at the horizon line. Keep it level! (There's nothing more distracting than a sloping sea.) Try not to have the bow of your kayak in every shot! Make sure to shoot horizontals and verticals in the same scene. If you want to illustrate a story for a magazine, vertical format is often preferred.

Quality of Light
Most people go sea kayaking during the middle of the day. As it's often warmer and lighter, there is good reason for this! However, midday is often the most difficult time to get great pictures. Caps and hats shade faces and the light is harsh.

To make really good sea kayaking images, try to shoot some pictures early or late in the day. Colors are more saturated, and shadows are often softer and more predictable. While grey days are not the best for brilliant colors, the drama added by rain, somber faces and thundery skies can work well.

"Try shooting pictures early or late in the day; colors are more saturated and shadows are often softer..."

Psychological Lines
When we look at a two-dimensional image, we make inferences about what is happening—these are called psychological lines. Psychological lines exist between the kayak and the sea/earth, between the person and the environment and between people in the same scene. Skillfully crafted psychological lines keep the interest alive in an image by providing a pathway for the eye to travel along.

When we “freeze” action it is deduced/inferred, consequent or subsequent action which really brings the image alive. If another human is pointing or looking at something we can't help but follow the direction of interest. What are those people looking at? Where is that kayak going to land? What communication is going on between the people? These are the types of questions you can easily build into a shot by understanding and using psychological lines.

Unless you are a competent landscape photographer, it is the human interest that makes sea kayaking images successful or not. So include people.

A simple approach is to borrow from photojournalism rules and apply the formula of: People + Background + Interaction = Interesting Picture.