I think I found the answer on one of the self service tech pages. It says power consumption with backlight on is .75 amp so that should give me a good full day on the water and a second charged battery is small enough to carry on trips.
I have made many FF/GPS portable and the 7AH batteries needed to be replaced often. You most likely will want to go with a larger than 7 AH battery. The GPS will draw it below 12V near 3/4 of day fishing. When those little 7AH battery get drawn to 11V a few times they are about toast. Around 12AH will get you through full day and end up around 12V.