it's the gain. Every 3db is double the gain. With a 9db antenna, you can expect no more than 20/25 if you are not rocking and rolling around and the receiver has his/her antenna on a tower like Tow Boat US/ USCG etc. Ship to ship would be less. I think it can be made better, but not with out of the box uncalibrated stuff like most boats have.
You can get an antenna that also supports a cell phone as well.
As endtuition says, higher Db gain will improve commo. The practical fact of the matter is that anything higher than 6Db on a small boat is nearly useless as the main communications antenna.
The transmitted signal around a vertical whip antenna is in the form of a torus or doughnut shape. The higher the gain, the flatter or thinner the torus until it would more resemble a CD than a doughnut.
A 9Db antenna on a second radio would be acceptable. You would, theoretically be able to transmit and receive farther, but you give up reliability due to the motion of the boat. Pitch and roll aim the signal at the water or the sky.
Very high gain antennas are best used on a stationary location, as a tower on land or an oil rig.
Digital is the way to go and my second choice like dutch said would be the Galaxy. I onced talked to a boat at the end of the TC Dike from 39 miles offshore with a 8' galaxy that was mounted on the gunwale of a 25' Mako. That however is not the norm, I think the moon and stars need to be alligned
properly to get that distance all the time. That radio is one of your most important pieces of equipment offshore along with a cell phone. Allways do a radio check when you break the jetties just make sure it's working.