Howzit 96720,
Sure, I did ton's of deep ocean surveys, but our boat was a bit bigger than what you're using (I was working for SOEST / U of H).
You can see the results of our work here:
http://www.soest.hawaii.edu/HMRG/Multibeam/products.php#ChartsUnless you're thinking about mapping inside Hilo Bay I don't think any of these consumer grade sonars will work very well. The waters off Big Island are just too deep. The good thing is that all that lava rock and coral probably makes for a good bottom to scan. Where exactly are you're interested? There's been tons of multibeam sonar scanning done along all the islands coastlines'. We went about as shallow as we could, sometimes as close as 25' to 30' depth, which considering the ship draws around 24' is calling it close. That website I linked to will get you started, but there's tons of other info around as well.
Used to live in Hilo, still have family (inlaws) there.
Aloha,
BigK