>>480062Look at the down imaging (DI) models.
I use a small home security battery (4"x3") inside a waterproof box. The cheapest method is from radioshack they sell battery holders for rc stuff. The 8 AA setup works for around 5 hours and is very compact.
you can mount the transducer several ways. youtube it. its super simple to do with goop.
I too will recommend a SOT for fishing.