With all due respect the fixed blade requirement is pretty much old school. Todays folding knives, the better ones, are mainly one handed open and not substantially slower to deploy or less stable or durable than your average fixed blade.

You might wish to look into the available folders and get some experience handling them and deploying them one handed. In an adaptation of good SCUBA and swimmer training, under stress if you can manage it.

A good test would be to arrange for a weighted line to be tied to your ankle and then have a friend near-drown you in the deep end of a pool. The weight makes getting air difficult but not impossible. Then your friend repeatedly shoves your head under water. Until you start to panic. Which usually happens after you get a lung full of water. At which point you are to deploy the knife, cut the line and self-rescue. This is good training for anyone who will be around water.

Of course you need a pool and a friend, or friends (even better), who are able to rescue you if you get into real trouble.

You may still decide you simply have to have a fixed blade but there are trade-offs. Fixed blade knives are fairly bulky. Additionally so because they demand a very substantial sheath with a strong retention system if your going to prevent damage to yourself, others, or vital equipment. The retention system can be as problematic when your stressed and inverted as folder. Both folder and fixed blades need a retention lanyard in case they are dropped while being used. Before being deployed a folder is much less likely to come out of its sheath and lay open a major vein while your spinning around on the bottom of a hole.

Neck knives are good in some situations but not so good in others. Unless you use a second retaining lanyard lower on the chest that holds the knife in position they wander around a lot. If your inverted they can fall completely off and be lost. After being thrashed in whitewater or falling down a hillside your as likely to find them hanging between your shoulder blades as around the front.

Fixed blade knives have some good uses in marine use. Something like this:
http://www.defender.com/product.jsp?path=-1|118|119|312085&id=78450

Is particularly good for places where you can lash the sheath in place at locations where a knife might be vital. Like the bitter-end lashings on your anchors rode, at the base of the mast and lashed onto the bindings of the life raft.

From personal experience I have found over time that even my favorite fixed blade knives are increasingly being left behind. There are precious few jobs a good folder can't do and unlike a fixed blade they don't get in my way. Like a good friend a fine folder stays out of my way until I need them.