Frank got it right.And why do all the dive shops have $25.00 crap knives
You'll also notice that they are usaully bright and shiny. Thats because many of them are made from 304 or 316 Stainless Steel that wont rust but it is way too soft to hold and edge for more than 2 minutes. Any good knife, even one that is made from 440 SS or any stainless that can hold an edge WILL rust.
Some of them dont even have sharpened edges, they are used more to pry than cut.
The SOG SEAL is a good knife. A "proper"dive knife will have a sheath that attaches to the leg.
A knife is a good thing to have...one should never enter the water without one.
I dove on a WW2 wreck in the Philiphines and got fouled on an old fishing net that had gotten hung up in it. If I hadnt had a knife to cut my way out...I'd still be there. As it was, I used up most of my air to get out of it.