Realistically you want around a 13-14ft narrower kayak, not a wide one. The Scupper Pro TW is the all time classic kayak, and now made under a different brand. OK Scrambler in the 13ft range would work along with many other brands.
As to GPS there really isn't an underwater unit. A nice handheld will do just fine, and Garmin even builds one with marine maps built in. I have a handheld depth sounder I sometimes use with my kayak as we can rarely see the bottom. More and more people are using water proof housings for their phones or iPads there days.
There are a few good threads on this board if you search the word "kayak".
My thoughts are though, unless you have a buddy with a kayak an inflatable boat or a scooter are more useful.