if you git die hard you could try it all, but i would just write down the temps for now, and check every diffrent part of the lake, or river, i use a swimming pool thermostat, $5. and it lets me know the main temp of the river, but if i git into some back water i can recheck it again...most fish finders have a serface temp and those are cheaper every day.
don't forgit a lake does not spawn all at once, it could take along time, for all the temps to come up, like by the dam could stay cold longer cuz it is wind blown and protected by sun light? or last month that far north end of hidden back watrer area could of spawned already cuz it gits no wind and full sun? temps dont lie but your thermostat could be off>LOL