It is all a question of the reason for carrying the tool. It is hard pop a rock scallop off a rock, smash open a sea urchin, or coax an uncooperative crap with a 3" blade. Small knives are also difficult for large gloved hands to grasp — handles twice the length of the blade just look wrong.
Depending on what you want to cut (assuming that is purpose), it is difficult to impossible to get and keep a good edge. The definition of a good edge varies a lot, but none of the "diving knives" are capable of a edge expected by a chef. Sure you can carry a kitchen boning knife, but the really fine edge won't last in salt water unless you sharpen after every exposure. Titanium and stainless grades used in dive knives are great for corrosion resistance, but can't develop and hold an edge your fish monger would use. Cutting edges are all about metallurgical compromises.
Short blades are awkward for cutting line much larger than ½". A Z-knife is probably better for mono-filament and fishing nets, 5"+ blades work better on a cargo net, and hack saws are better for 2" Polypro. Hammers are better for pounding nails than pipe wrenches. All tools are defined by minimizing compromises for their task.
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