How does a portable fish finder work?

I am taking a fishing trip to Ontario this August for Pike and Walleye and will be renting a plain, 12-foot aluminum boat instead of taking mine up. Mainly for walleye I would love to know the depth I will be fishing and am looking into portable fish finders. Basically, I am confused as to how the sensor works... do you actually throw it in the water and it goes down to the bottom then tells you or throw it out over the water and it bounces sonars to the bottom, also could you just set it in the bottom of the boat to get readouts? Thanks in advance.