I have a friend that uses an inflatable yak, he bought it at the boat show at Excel, they told him you could fish safely from it , he's had it a couple of years now and todate he's had no problems with it, but he's only used it around the creeks of Essex chasing mullet, main reason he bought it was so he could drag it about from creek to creek , he has had a few sorties around the local canal , one of the main problems with it on the canal , is windage, any sort of breeze and it tracks in the water, I also have to point out he mainly uses it , to get himself to his desired spot , then fishes from shore or mud bank.
Personally I would'nt bother with them for fishing, they are severly limited when it comes to the sea , and in my opinion downright dangerous in any sort of open water, you might do better with a float tube.