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This is the "need to have vs. nice to have" discussion. For use on a lake, a depth finder is pretty much the only thing you really need to have. A VHF radio may or may not be useful on a lake, a cell phone is probably as good for summoning help unless the lake is in a very remote area with no nearby towers. GPS will make a good speedometer but will most likely not have any charts of a lake loaded onto it. Be sure to have a compass in the event you are caught in fog or darkness and need to know your course. Get a map of the lake and you should be fine.

Unless a lake is large enough that you can get to a point with 360 degree horizon, electronics are not really that big of an issue.