Diving knives must be made of corrosion-resistant metal, typically titanium or stainless steel. If you’re looking for something that doesn’t break the bank, you should go with stainless steel, while if you’re looking for something that’s super durable and effective, opt for titanium. Bear in mind, however, that titanium is harder to sharpen since it’s a lot more corrosion-resistant.
By all means, you should always rinse your knife after each dive to extend its lifespan and durability.
As for the handle, make sure that it feels comfortable and firm in your grip. While rubber-coated ones are better for grip, they may not be as comfortable as plastic to hold. Finally, rope-bound ones are the most superior in terms of grip.