Pjard;
Antenna length is the biggest factor in determining transmission distance. Going from a 4' to an 8' antenna will double your db gain or transmission distance. Here's a link that explains it better...
http://sierracgaux.org/vhfradios.html
As an fyi I have an 8' antenna mounted in the center of my rod holder tube on the top of my Eastern so effectively I am approximately 15' above the water. This gives all the transmission distance I need on the lake and then some. If you do get an antenna, make sure to mount it with a quick release style mount to lower and remove quickly that is how mine is mounted. See pics below.
Good luck!
Dan