Fish finder

[Emptyhanded]

I am looking for something i can use in my 10' jon boat to measure depth of the water when i am trolling. I have been looking at a HawkEye portable fish finder but i am not sure if that is the way i should. I don't want to mount a permanent fish finder because i haul it in the back of my truck and i don't want to screw anything up like the sensor. So what is my best option for knowing the depth for trolling as far as a portable unit goes? Thank you.

Jesse