I am also pretty new to kayak fishing, but I have found my depth finder to be very valuable.
It takes some getting used to, specifically reading what you're seeing. It helps to adjust the settings a lot.
It is very nice being able to find the structure and terrain fish relate to. You can do the same thing without a fish finder by fishing a location frequently, the way old folks used to
, but it is much faster to use the sounder.
For me, the most important feature in addition to the sonar was GPS. Being able to get right back on top of a location after you find it is a huge time saver.
I find now though that being able to record my trips to an SD card and view them on my computer really has helped me learn to interpret what I am seeing. It also lets me go back over and see things I missed(which I do all the time) and mark structure on the GPS after the fact.
As for Texoma, I am very interested in other people's response to that question. I live in NDFW and would like to head up to Texoma some time with the kayak but I've never fished there, let alone fished there from a kayak.