Viewpoints can be highly limited. On a ship you may be far above the water's surface. In a zodiac or raft, you're right at the water. Water can change light conditions, and its movement can affect the photographer's ability to capture the image desired.
The photographer's ability to transmit the scale of the image is often impared in water shots as the frame of reference relevant to the photograph's viewer is missing or unfamiliar. In addition, the environment of being in a boat or on a ship can be hostile to your photographic equipment, and taking photographs.