My understanding of antenna gain, a higher gain antenna focuses more of the signal in a narrower, horizontal beam. So the
power isn't increased, just more of the
power directed in the horizontal plane.
The downside is the beam is narrower so if the
boat is heeled or rocking a lot the signal from your antenna may no longer point where you want it.
If you will only use it at
anchor, your
boat is reasonably stable, etc then higher gain would probably work for you.