I would find a cheap water Prof fish/depth finder.... The reason I say this is 99% of the time your going to use it to find deep water or holes....The Kayak could be in a foot of water and you would never now it with out the use of the depth finder...
I paid $75.00 for mine and had it now for years and its still as good as the day I put it in.
So after you find out what your going to get... then make up your mind what would be a good place to mount it so its out of your way.
If you need help with that let me know and I will walk you through..... its easy....
Hope this helps some........