Kayaks are an amazing medium for wildlife photography. You are down low, giving an almost eye to eye view with your subjects. You can move in almost any direction, there are no trails you have to worry about stepping off of (although a river or slough could be considered a really wide trail.) There are good views of what is around you, no trees or buildings blocking the views, giving you an idea of where to go and who is around to be photographed. However, due to its nature kayak photography also has some special considerations, many of which can be considered as pros or cons, depending on how you deal with them. I try to make them benefits.
With any wildlife photography it is important to understand that you are seeing wild animals. Their lives are fraught with danger and they don't understand that we just want to shoot them with a camera and not a gun. They don't realize we are creeping closer just to see them better, that we won't suddenly pounce on them and carry them off. They need us to respect them and keep some distance. Stress is harder on them- there are animals that can literally be frightened to death. So please, even though there are no physical obstacles between you and that seal when you are on the water, give them their space. They will fly away or swim away if you approach them incautiously or too closely, losing any chance for that great photo. Also, it is against Federal Law in the U.S. to approach and disturb marine mammals.
That said, there is some good news. It is not illegal for these same animals to approach you, and there are a lot of curious marine mammals out there. If you do give them their space, many times they will actually approach you, because they want to figure out what you are. Otters, seals, dolphins, even whales, can all get very curious and decide to come close enough for some amazing pictures (whichever camera you are using.) Seals especially love to pop up right behind kayaks where they think you won't see them. Just remember to let them come to you. The Sea Lion to the left, and the Harbor Seal below did exactly that.