Lots of good quality folders have been mentioned here and I own one or two of several of the brands mentioned like a Benchmade AFCK, several Spyderco's and a CRKT. Before I started working in a place were I have to go through security every day, I had one of them in my pocket every day. Still feel naked without, but on the boat I have a cheap West Marine sheepfoot type blade for a folder. Its nowhere near as durable a knife as a Spyderco or Benchmade but it gets the job done and is no big deal if it goes overboard. Good quality locking folders with one hand opening are not cheap and an equivalent automatic would be a good deal more expensive than the thumb opener.
That said it would probably be a better idea to have a couple of decent fixed blade dive knives stowed at the cockpit and mast rather than a folder in your pocket though.
s/v Palmetto Moon
1991 Catalina 36