I'd check the transducer/unit connections. That seems to be the cause of majority of these weird problems.
My fishfinder has been acting up lately too..
It works fine a couple of hours, than just looses it - it is on, but it can't read squat... No bottom reading, nothing... It's not the battery, it does this with 3 different batteries... I guess the new one is order.
Salt water is taxing these electronics on the kayak big time.