Marine antennas are typically high gain omnidirectional antennas (6-9dBi) to allow maximum off shore range no matter which direction your vessel faces. Selecting an appropriate antenna depends on the type of vessel you have, and how far off shore you plan on going. As a simple rule of thumb, the higher the antenna gain the further off shore you can go. 8 or 9dBi marine antennas can easily extend your range up to 120km off the coast. However because higher gain antennas have a smaller angle in which they can receive signal, smaller vessels that sway side-to-side may be better off with a slightly lower gain (such as a 7.5dBi) antenna to compensate for the motion.
In practice this is generally not an issue for most vessels - the receiving angle is a 'half power' measure, meaning that if your boat was to swing past the quoted angle, the received gain would be 3dB less (half the power in Watts), the same as a lower gain antenna anyway.