Last year I bought my first kayak. It was an Ascend D10 from basspro shops. It was a sit in kayak. I never once felt unstable out on the water in it. Not ever. I got wet twice with it. Once I went in the water getting in the boat and once getting out of the boat. I estimate that I put maybe 200 hours of use on the boat. (I get that number from about 40 weekends at 5 hours each time.) So using the boat that much in all different types of weather, getting wet twice is a good number to me. Now I never did anything stupid such as going out in the middle of the lake with idiot jet skiers flying around or pleasure boaters that don't know common courtesy either. But I did several lake trips and stayed in coves or went to "no wake" lakes. I also did several trips to the Duck river near shelbyville and handled the areas that had some minor rapids really well.
There is no way I would ever attempt to stand up in that boat. However, it was not designed for that. This years same model of that boat at BassPro has a tunnel hull and a much more comfortable seat.
http://www.basspro.com/Ascend-D10-Si...5012221/212172 So yeah you probably could stand in that if you are brave.
This year I have upgraded to a Native Watercraft Ultimate 12.
http://www.nativewatercraft.com/boat.cfm?id=5 It's best described as a cross between a sit in and sit on. I've only taken it out 4 times so far. Is it better than my Ascend? Absolutely! But the difference is I spent 1000 bucks on this one where as I only spent 400 on the other so it better be.
Now I only have 1 year of experience kayaking. With that said, if my opinion was asked, my answer is buy what you can afford. If you want a sit in then buy one. Chances are most people that are on the hype of how much greater a "sit on" is verses a "sit in", have never owned and paddled both. As I have never owned a sit on I can tell you about my experience with a sit in. It was an enjoyable one. Whatever decision you make will be the correct one. There is absolutely nothing more fun when it comes to fishing than fishing from a kayak.