Welcome to ribnet.
I guess the big question is what lakes you want to survey. If as you say "millponds" then that setup will work fine, (or rowing might work better!) as the electrics are (can someone confirm) infinitely adjustable as to the depth they sit in the water, however if, say you were planning on surveying (and I use this as an exaggerated example) Lake Windermere, you're going to need more than a couple of batteries!
Some food for thought:
- A sounder will draw a fair current. So will an electric outboard. If you go "pure electric" you are going to need some hefty batteries.
- As you have said, some places ban "fuelled" engines
- I am not aware of any recording Sounders - I assume you also have a laptop of some description to combine the depth & positional data - that's going to need power too....
- said assumed laptop will need to be kept in a watertight bag!
Not knowing what lakes etc you are planning on surveying, I would guess for a small "puddle" your proposed setup could work well - Laptop might last 4 hrs or so, and the "car batteries" keep you moving and the sounder alive.
I would think if you were surveing anything bigger than about 1/4 mile in any one direction you might want a "yacht" type outboard - small but with battery charging. Likewise you might also want to up the size of your a little as 260 might seem big in the chandler's showroom, but once you are out there with batteres , kit etc a 2.6 m is going to seem quite small.
There is an argument for a secondhand Avon transom dinghy. They have Ply floors, so you could potentially screw a couple of battens through from below to the ply to (having of course used CSK head screws and CSK'ed the ply floor to take them!) then use as a base for building a "workstation" for the laptop. Nothing to stop you doing that to any other dinghy tho', but you might need to build the ply floor first!
Couple of links for you:
Bathymetrical Survey of the Fresh-Water Lochs of Scotland, 1897-1909 - Maps - National Library of Scotland
Rich gent decided to do exactly this in 18 canteen with a lead line and a wooden rowing boat...... The resulting charts might give you an idea of the different scenarios you might find (e.g vast areas of reeds you could measure the depth with a school ruler)
oops. Lost the other bookmark. Search google for this kind of thing , I found an interesting website talking about the electronic tech and other surveying techniques (like deciding how to drive around to get the best profile of the bottom and not miss interesting features).