Hi There to all kayak fisherman's.
I am not a pure kayaker (I use the paddle just to launch and as a second option for propulsion). I use a small outboard stinker 2.5Hp 4 stroke on my kayak, planning to upgrade to 3.5Hp this year.
This allow me to go deep on Erie, Ontario, Simcoe or G Bay.
I love trolling early in the morning alone on the big water surface of great lakes.

[YOUTUBE]https://youtu.be/gjxG1EmZFy8[/YOUTUBE]
My kayak is a Wavewalk S4, it does come with a built in transom and can take up to 9.9Hp if you want to get nuts on water. Recommended power 5 to 6Hp.
I do prefer the 3.5Hp because give me the ideal trolling speed of 2mph in idle mode (throttle at minimum).
The hull of this skiff is designed as the hull of a catamaran, very stable.
In top of that it is easy to mount a fish finder in front and have the transom well placed in the back.
The saddle it is filled with air, the skiff will not sink if is filled with water.
For small rivers or lakes I use a Minkota 80Lb / 24V thrust on two AGM batteries.
Have fun and be safe.