When it comes to buying a dive knife, you have to consider a few factors. For starters, it is important to remember that a dive knife is not intended to be used as a weapon, but a tool. As such, there are different factors to consider when buying a blade for diving than if you were buying a blade for hunting or for self-defense purposes. A diving knife is used for cutting line, rope, netting or kelp; for adjusting or repairing equipment; and even for signaling, with the butt end.
Diving knives generally come in two varieties: Stainless Steel and Titanium. So what is the difference, and which is better suited for the intended function and environment in which it will be used? Let's break down the main factors worth considering when looking for the ideal tool: hardness, strength, corrosion resistance, weight and cost.
Titanium is stronger, has higher corrosion resistance, and has about half the density (weight) of steel. Steel is harder (meaning that it will stay sharper for longer) and is less expensive than titanium. Some blades have a titanium coating to offer a balance between the two. The titanium coating helps strengthen the blade and prevent corrosion, and after being sharpened a few times it will expose the steel edge underneath, giving you the best edge with higher strength and resistance to corrosion over the length of the blade. Check the product description of any titanium knife to see if the construction is titanium (or titanium alloy), or steel with a titanium coating.
So which is better for diving? In my opinion, the best knife for diving is probably a titanium alloy that has a hardness scale rating closer to steel (something with a rating of at least HRC 50¹, such as this one), but such knives are much more expensive than stainless steel knives. In the end it will be up to you and your wallet which knife is best for your circumstances. You might also consider carrying one of each: a general purpose titanium utility knife, and a stainless steel cutting knife.
¹ For comparison, stainless steels used in blade construction are usually rated between HRC 55-66, depending on carbon content.