I made my own composite SOT many years ago before you could buy the plastic ones that are so popular today. I say SOT, but it has a deep self draining pod that you sit in more like a sea kayak without a cockpit. This gives it stability without the need for a wider, slower hull. With a lap strap made from a divers quick release weight belt across the thighs it is possible to use all conventional boat and paddle skills including rolling. At 17' 6" she is as fast as any sea kayak. I originally designed her so that I could free dive /spear fish in areas that were unreachable by swimming or long journeys by boat. As it turned out so good to paddle, I ended up regularly paddling long daily trips exploring the stunning SW coast. Although I now also paddle conventional sea kayaks, I still resort to my SOT for paddle and dive trips and feel safer paddling this kayak solo.
So what are the advantages of a SOT? As I become more confident in sit in kayaks there is not much to choose between them except what I am intending to do. The SOT is IMHO a more versatile craft as a tool to do a job (fishing, diving) as well as simply enjoying a paddle. With good breathable dry suits, they have no real disadvantage in principle. Having said that, most SOTs are not like mine :)) They tend to be considerably slower over a distance and many give up their claimed stability by sitting the paddler above the water. Just this weekend I paddled out for a play in the Old Harry Race off Poole with my son. It was a spring tide with some nice standing waves. As we came back in, I witnessed 3 SOT fishermen fall off their kayaks within 30 min of each other. This was on the edge of the race in slightly confused, but relatively flat water. I went to have a chat. 2 Prowler 13s and a Tekport They were all relatively inexperienced and had over estimated the stability of their craft, but at least they were well equipped and dressed for immersion.
I do think that excellent Kayak Fishing Forums such as Anglers Afloat have done a great deal to educate at a time when BCU and traditional channels seemed disinterested and dare I say looked down on SOTs. Years ago, this lead to me often paddling on my own and finding my own way as I did not fit in with the way others did things. Times have changed now of course and I am quite proud of my craft that always raises interest from both schools of thought. There does seem to be an increasing number SOT paddlers moving into conventional kayaks, but keeping the advantage of having both options.