After doing a lot of research over a year ago now I found this one:
Rescue Knife Floating (Not Locking) - Yellow
It's cheap, razor sharp and it floats too
I think you can get glow in the dark ones too.
I have seen them in orange and yellow.
Plastimo make them as well.
They fit in the musto pouch that comes with their trousers so I removed the pouch and put it on my lifejacket belt
Been using it for over a year and it's great, it's right up there with the Aldi grab bags