I have a garmin E-map handheld That I'm attaching to a Gaffrig performance GPS head unit.
I don't have the boat going yet but from what I'm told this should work perfect.
I have used the E-map in the car and it does go over 100, but that is a differant story.
There are several things that you need to have for it to adapt to the gps speedo head unit on your boat so you should call the manufactor before you buy
Good Luck