You really should consider a fixed blade, kydex sheath and attach it to your PFD.
If you really need your knife in an emergency, you don't want to be fiddling with opening it up (spyderhole or not). One handed opening a wet knife with cold wet hands in confusing (i.e. underwater) circumstances is bound to lead to a dropped knife.
There are a number of good options for fixed bladed dive knives. The Gerber river knife is a good cheap one. Benchmade has a dive knife (made of X15) that is corrosion resistant; Spyderco caspian salt and the aqua salt are both in H1. All come with a nice kydex sheath.
I do a lot of boat work myself. I used to use a gerber river knife. I lost it though. I now carry a cheap buck diamond back guide (stainless steel) on my PFD strap. Eventually will go for the Spyderco aqua salt or benchmade dive knife. I'm leaning more towards the benchmade because it is a bit more compact (3.5" blade versus 4").