Solar on the kayak definitely wouldn't be my cup of tea, but the technology is available...
Hobie has a foldable 23W Solar panel for the eVolve that kicks out enough juice to run the motor and push the kayak at about 2 knots indefinitely in sunny conditions. It weighs only 1.4 lbs and is designed to withstand getting wet with saltwater. With a few mods, I bet that could run a fishfinder or 2.
On our Santa Cruz camping trip, I was able to run my Garmin for 3 days off the same 12V 12aH LiFePO3 battery the entire trip without needing a charge. For an extended trip I think my preference would be to just bring a 2nd battery and KISS.
