There is a handheld depth sounder on the market
It was covered in a pbo issue early last year.it just a small spot about new bits from the boat show, i think,
useful for going ashore in reefy areas it said.
i think it was about 48 squid so not cheap, probably doesn't float either so attch a bit of cork so someonelse can pick it up on the tide when you inevitably drop it.
have no more details than that might have seen it in the gaelforce catalogue
SM-5 Speedtech Depth Mate, £113.94 inc. vat and postage according to the invoice for the one I sold last week. Looks like a yellow waterproof torch, stick it in the water pointing down and there you are.
It's not on my web site, I'm so far behind with the new product loading, but please e-mail me if not frightened by price!
Not internally powered, but a lot of dragons racing on the east coast tend to use NASA Target or Clipper (only real difference I can find is the display) echo sounders. All we use for power is 2 heavy duty 6V dry cells which tend to last us most of a season (racing easter to October, 2 races per weekend, generally a minimum of 3 hrs per race to give you an idea of time). There are various other batteries rigged to them within our fleet including some that are rechargeable. No idea of price of either of these as we have had ours a few years.
Seafarer 701 has revolving led and digital displayplus shallow and deep water alarms, similar to nasa stingray.Can be powered by 12 volt battery or 6 x 1.5 volt AA batteries fitted internaly, standard transducer ( through the hull or internal tube mount.
don't know if they are still made as i bought mine from a car boot for £6
try contacting solent electronics on 012020301316 (seafarer specialists)
Lowrance/Eagle used to produce a portable fishfinder (FAR superior to a depth sounder) with a transducer on a suction cup. I can't remember how it was powered but it's worth looking at.