I would really look into a mid length (maybe around 10 ft) Dagger or Old Town. Their prices are pretty realistic on a lot of models, and both brands that I have had, have held up well to salt and fresh water. Cabella's has a good selection imo.
Keep the kayaks clean too. Wash them if you can after each trip- especially if using in salt water. Oh- and avoid oyster reefs if possible- lol.
Too short- and I think you will spend a lot of time paddling to make much headway and to keep it going straight.
Too long- and they can become cumbersome in transporting, carrying, and in water that has tight areas due to marshes, trees, or other obstructions.
I personally like one that is light enough to lift and carry with one person- although that might not matter much to you- different strokes.
I recommend while (or shortly after) getting your kayak, to also look into a very comfortable and well fitting USCG approved life-jacket, and an ergonomic paddle. Bending-Branches make great paddles imo.
I agree with SONIC on the "jack of all trades" dilemma, however if you go specific (long touring or short whitewater) then you will limit yourself to being able to go anywhere. My mid length Dagger isn't the best on open water, or on tight rapids, but I like it. I like being able to take it anywhere, and I have had a ton of fun with it in many different situations- ocean, in-shore bays, lakes, swamps, bayous, rivers, etc.
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