Good choices for a water knife, at very reasonable prices are the gerber river runner and the gerber river shorty. I own the river runner. It's 3 inch blade is partially serrated with a sharp tip (river shorty is all serrated with a blunt tip) with a good grip.

And the sheath is excellent. Holds the knife securely, but it comes out with a tug, and allows for multiple carry options. You wouldn't want to shave with the blade, but for most purposes it's sharp enough.

And the price is right - including shipping on either amazon or ebay the cost is about $27. I've taken it with me on hikes and it's performed fine. I haven't gone scuba diving with it (yet) but it will function fine as a dive knife.

A very fine neck knife is the buck mayo kaala - one piece of S30V steel, three inch blade, and again a good sheath. Unlike many neck knives, this one won't fall out. I got lucky and got mine awhile ago on ebay for about $40, including shipping.