Most VHF Marine Band radio antennas for use on small boats are a half-wavelength in length, or about 37-inches long. In some cases the antenna is contained in a much longer fiberglass tube, sometimes as long as eight feet.
The range of a VHF Marine Band radio is primarily determined by the height of the antenna above the water level. To improve range, increase the height.
Antenna gain is always welcome, but it comes with a price. The radiation pattern of an antenna narrows as its gain increases. In the best case the main lobe of the antenna is oriented in the most useful direction. In small boats which are often in complex motion when in a seaway, the antenna orientation may not always be optimum, and the main lobe of the antenna may not always be pointed in the most advantageous way. In these situations, an antenna with slightly less gain but a broader main lobe may prove to be more effective. Antennas with gain also have deep nulls in their pattern, and if you happen to have the antenna oriented so the desired direction of transmission falls into a deep null, the signal will be reduced, not increased, as a result.
Marine antenna makers are extremely generous is allocating gain to their antennas, and in most cases claims of gain are exaggerated. Marine antenna makers are extremely shy about publishing any information about the radiation pattern of their antennas, leading to some skepticism about how clean those patterns are and how many spurious lobes and nulls they contain.
For a discussion of antenna gain and pattern as it relates to the vertical monopole antennas used in the VHF Marine Band, see
VHF Marine Band Antennas
http://continuouswave.com/whaler/reference/antennaVertical.html
For a discussion of how to use remote NOAA Weather Radio broadcast stations as a known signal to check radio receiver and antenna performance, see
Assessing VHF Marine Band Antenna Performance Using Remote NOAA Weather Radio Broadcasts
http://continuouswave.com/whaler/reference/antennaRange.html
For my own small boat I use a three-foot antenna which I have mounted on a four-foot extension mast. This combination gives me the highest antenna mounting that is reasonable on my boat without a complex mechanical support for the mast. I use an antenna with an exposed metal radiator. This antenna can be replaced in component parts. Unlike an expensive eight-foot fiberglass antenna when you break the top of it, you won't have to discard the whole antenna if you hit something. Actually, because the antenna is a metal whip, you probably won't break anything. I described my installation in detail in
VHF Marine Band Antennas For Small Boats
http://continuouswave.com/whaler/reference/VHFAntenna.html
I have no reservations about recommending this type of installation to others, and many have adopted it with good results.