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.