Looking for suggestions for a new sonar/gps for my yak. I could spend days researching but I'd rather be fishing. Need decent fish finding ability, and very important that I be able to download/buy super detailed Texas coastal topo maps, and Texas lake and river maps. Going to mount shoot through, like my current unit, and run on a deer feeder 12V battery. I'd like to keep the total investment at $600 or less. Thanks in advance for any suggestions as to units recommended or those to avoid.

Humminbird is the only manufacturer that makes a kayak specific install kit.

http://store.humminbird.com/products/356206/385ci_Combo_Kayak

See the video for the installation kit at the bottom of the page.
Meets your budget and is sold by ACK (good people, excellent service).

http://www.austinkayak.com/products/2637...Fishfinder.html

Color, GPS and Maps are upgradeable

BTW I have a Humminbird PiranhaMax 180 and I use it mostly to find structure to fish not fish. And just because there are fish there, doesn't mean you will catch them or that you would want to. There is no species ID and size is pretty generic.
I troll running contours with my FF and that has proved very productive. Last 2 trips produced a nice keeper Hybrid (each) a Crappie ((each and yeh, trolling) and 20 & 21 White Bass. I have caught way over 200 fish already this year.

The 385Ci is a very nice unit and will accept Map cards from Navionics and others. Navionics and the Texas coastal maps are great. I use them both in a larger Humminbird I have installed on my power boat (998Si)

As for the best prices, check xtremenavigation.com. The best prices and free shipping that I have found. Actually better than my employee pricing at that place where I sometimes work... When I buy the 385ci (and it will be soon), I'll get it from these folks...

http://store.humminbird.com/products/356206/385ci_Combo_Kayak
I have this one, I think I put a Navionics hotmaps chip in it. Super detailed, works great, and I'm happy with it so far. It's being repaired right now at Humminbird for a transducer problem.

I use a Smartcast. lol

I've used a Humminbird 788ci with great success from my bass boat. Dunno how it might work from a kayak but if you are kayak fishing, I'd think it would do the trick. Nice features on that little bugger too.