All I care about is depth.
What do you recommend?

I have a el cheapo Humminbird PiranhaMax 170 for a cartopper and it works well. I think I paid $75 for the portable unit and bag. Matter of fact, I'm charging the battery right now for a weekend trip into the woods.

Pretty much impossible to find a single feature sonar unit these days. Hummingbird has a nice unobtrusive digital depth gauge.

http://www.gandermountain.com/modperl/product/details.cgi?i=80325

If you want to be able to run at speed, then just look for a fish finder that has "flasher mode".

I take it you mean structure too. On our lakes which run shallow and mostly not so big the best use is putting your boat over the most likely structure, points and dropoffs and so forth. The structure locations are commonly known, there are contour maps for many popular lakes, but getting on them at the right depth is difficult using landmarks.

Don't have a recommendation on a current production unit, my old stuff is fine for that.

Some days (and nights) I like having the chartplotter to see where I'm at.

You just don't get in a big hurry, but ya won't get out there and do circles.

If you are sure all you want is to see the bottom, consider something like the vexilar. It works on boats (with a different transducer), and can do double duty in the winter ice fishing. You don't get winter up there, eh?

It works, I use an old Hummingbird portable flasher for ice fishing. But in the boat the LCD screen is nice to see where you've been instead of keeping track by staring at the flasher. Easier to interpret how fast a dropoff drops or how fast a point rises. Besides, something to play with when things get slow. The optional speedometer is handy for maintaining consistent trolling speed. Funny how far off you can be going by feel and then looking at the readout particularly with a wind. Our walleyes can be fussy about trolling speed.

How about looking for a used one? Lots of guys just have to have the latest with color, GPS, charts, popcorn popper, whatever.

Somebody used to make a "portable" fish finder which just clamped on the side of the boat. Sucked for finding fish, but I found it worked pretty well for telling the depth. By using it to find the points and dropoffs I was able to catch fish with it. Best thing is it required no install on my 10' little aluminum boat and ran off of C batteries. Heck, it's probably in the garage still..

Ah, found it online. Bass Pro has it, the HummingBird 110. Priced at $130. New ones run on 6AA batteries.

http://www.basspro.com/Humminbird-110-Fishin-Buddy-Ultimate-Portable-Fishfinder/product/97460/

Would NEVER recommend it for any serious fishing, but if you don't want to lug around a car sized battery to power your fishfinder, then it's a decent option.