Hello,
A wise man once said if you want to keep anything from rusting/corroding then keep it OUT of saltwater!
The key to any cutting edge for diving is the material used. I.e. if you want a razor sharp edge then your forced to have higher concentrations of carbon, as we all know carbon is very easily broken down by saltwater. (quick test: drop a razor blade in salt water for a few mins., remove it and look at it the next day)
The secret is the right mixture of materials for the job at hand. I.e. sheers tends to have high amounts of carbon so they rust easily. Especially in the rivot. These titanium dive knives have problems keeping a good sharp edge (hence the word SHARP)
I have discovered that for my applications it is best to use a mixture of the two
I mostly use mine to cut fishing line, webbing (i.e. cut my buddies bc/wetsuit/weight belt/etc) and rope. I bought a pair of sheers, and a buck tiburon.
http://www.buckknives.com/products/details.php?ID=33 The interesting side to the tiburon is not just the metal but it has no classic handle, it's just a skeleton. Hence EASY to clean.
Another factor that comes into play is the tensile strength of the blade. I.e. use for prying or stabing?
Cleaning is another factor to consider. Do you want something that's high maintance or something you can neglect? Typicaly the higher carbon the more maintance you must do.
In short the trade offs are this:
High edge retention = high maintance and easily corroded/rusted
Low edge retention = low maintance and almost impossible to keep a good sharp edge
Low corrosion/rusting = hard to keep/hold an edge
High corrosion/rusting = very easy to keep/hold an edge
Hard blade = easily chiped/broken
Soft blade = easily bent
In order to get a perfect diving knife you need to answer all the above questions and what will you be using it for.
Hope this helps.
Ed