The hazards are many and diverse:
First, there's the danger of being unable to paddle back to shore, as a result of fatigue, and even exhaustion.
Strong wind and tidal current are external forces that could be hazardous to a tired kayak angler, especially if elderly or inexperienced. If your kayak makes you prematurely tired, you need to consider switching to something more comfortable that tracks and paddles better, namely a Wavewalk kayak.
Additionally, overheating in summer, and hypothermia in winter should not be underestimated, since they reduce your ability to paddle effectively and get back to your launching spot, and could even prevent you from getting back to shore.
If your kayak exposes you to cold wind and water, you should avoid paddling it under such circumstances, unless you're willing to spend hundreds of dollars on a dry suit. Remember: wearing waders and boots while operating small boats is very dangerous, since such heavy clothing might prevent you from swimming, and from getting back into your boat, or kayak.
Furthermore, there's the hazard of cramps, leg numbness and being practically paralyzed by pain in your back, or in your butt.
Cramps in your legs can be very painful, and last a long time if you can't stand up safely and relax. Sit-in and SOT kayaks offer only one position, which is sitting in an L shape, with your legs pointing forward and being restricted by footrests. Paddling or fishing in this position for a long time is s recipe for cramps, and for leg numbness, which is not as dramatic, but still most unpleasant, and undesirable as far as paddling is concerned.
Both leg pain and leg numbness are also dangerous because they prevent you from balancing and maneuvering your kayak efficiently.
Pain in your back or butt is dangerous, since it can literally cripple you, and compromise your paddling efforts. Moreover, you might find yourself close to shore and still unable to beach your kayak, or get out of it, as Don, this California kayak angler describes in his kayak review:
-"I fished for 8 years in an [Brand, Model]. The "L" sitting position and it's effect on my back is what finished standard kayaking for me... One day I beached the bow of that 16 footer and was still about 10-12 feet out in the water where I was sitting. I discovered I couldn't move my legs. Getting out of that thing without causing all kinds of laughter from spectators was one of my greatest physical accomplishments.
I was sure I'd avoid those scenarios with the"W", and I could hardly wait to find out all the wonderful differences."
Read this entire kayak review >>
This is not a rare example, and in fact many people report experiencing similar things with their sit-in and SOT kayaks.
Actually, most kayakers and kayak anglers have grown used to frequently stopping their paddling and fishing, just to go on shore an 'unkink'. This 'unkinking' basically means stretching, and allowing some rest for the sore back and aching legs. Stretching your legs and back improves circulation, and relaxes muscles and tendons.
Some kayak fishermen in warm regions just stand in the water, if it's shallow enough. If such frequent ergonomic stops are impossible, the only solution anglers face is to cut short their fishing trip, and paddle back to their launching spot.
The combination of back pain or leg pain and cold is dangerous because the cold further contracts your muscles, and thus increases the pain, and therefor makes it harder for you to paddle.
Paddling in intense heat, especially if you've suffered from a sun stroke or dehydration is dangerous if you have pains, because such combination can increase the danger of premature fatigue and exhaustion.
More on kayak fishing pains