I'm with Barrell, I use a Lowrance X50ds. Cheap yes but good enough. Follow the advice offered by Barrell and you should have the device for a long time.
I use a battery that is meant for emergency lights. It is sealed and lasts forever as far as run time. Recharge it and your good to go again in no time.
Most of the time I do not rely on the FF portion but if you do a lot of night time fishing in places you are not sure of it comes in handy to let you know it is shallow enough to stand up in.
Other than that KISS, as it will get wet and corrosion will make its way into the equation.
Happy Catching
Paul