I think it's a great idea for the price. However, I would have a few concerns:

-size of my iPhone screen: it's small, may not be good for seeing detail. iPad would be better.

-battery life of my iPhone wouldn't last too long. Maybe if I had a backup battery or a Goal Zero solar charger, it's be different.

What do you all think?

I have an iPad but they aren't water proof and it's hard to see the screen outdoors. Plus, you'd need a USB 2amp power supply and a mounting system. For the cost involved, you could get a waterproof, depth finder that is easy to read in the daylight that comes with a mounting bracket.

The one aspect that intrigues me most is the exceptional resolution of a tablet versus just about any sonar unit out there. The definition you'd get would be outstanding.