Geno,
I too am a newbie on the board and also working with the challenges of a party barge. I just put an "experienced" 80 model Johnson 115 on my 26' barge. Replaced a much newer 70 and am kicking myself that I didn't do this five years ago. That's what this old barge really needed. The speed the 70 achieved I can now do running only 4000 rpm. Full throttle gave me an increase of almost 50% in speed. I like cruising with the motor loafing along and you can carry on a normal conversation with folks on board.
My instillation on the left "toon" is pretty much blocked by the lower unit of the motor. I still get usable (for my purposes) info on the right at low speeds, but that might not be sufficient for the info you need. In the next week or two I will be adding a third "toon" to my boat and probably will move the si transducer to the rear of it just in front of the motor. If your boat turns out like mine you will likely have to install two sidelooking transducers, one on the rear of each "toon". I recently purchased a HB 798ci si HD, kinda the "poor boy's si", but that does not use the higher definition 800Khz transducer that would likely be a better choice for you. I think that is standard with the 900 and above HB units and optional on the 800 series. Not available with my unit. You might also think of mounting a transducer in front of the barge using a folding support that you can deploy when you need it and fold it up safely out of the water otherwise.
I am a bit your senior. (Think lower 70s). Taught computer programming in local college for a number of years and retired to operate a computer business then retired in '02 when wife had breast cancer. She's fine now and I have time freed up for hunting, fishing and diving, and drooling over sonar pictures.
The wide availability of good sonar units with side imaging and high definition downlooking capability, even 360 coverage are out there in stock at your local marina today. Check with some of the guys on their posts about bottom composition to pick out what will be needed to work for your project. You might also look at Reefmaster and Lakemaster Autochart to see if the bottom composition readouts available with their products might be helpful to you. Autochart is not currently available and has obviously no user reports available, but Reefmaster has been out for several years with lots of experienced users and several updates to make it a well established product. This discussion item on Reefmaster on this forum will likely be helpful to you:
http://forums.sideimagingsoft.com/index.php?topic=6950.0 A post on that section will get you a response from Reefmaster as to the adaptability of their mapping process to the arcGIS system.
I hope you will keep us well appraised of the progress of your project. It sounds like it would be a really interesting study. Good luck to you.
Chuck
P.S. That Tohatsu 90 should do a really good job for you. I have a good friend that runs a local marine repair shop. That is his motor of choice. Swears that it is more fuel efficient, easier to maintain, more trouble free and reliable than any of the 4 strokes. He has been using them on his personal boat for quite a number of years.