LogFAQs > #963050251

As a diver: Not well at all.

Any dive master will laugh at the idea of using a dive knife for this reason, and many are even blunted at the point (to kinda double as a flathead screwdriver/to open clamshells), and you a carry a dive knife primarily for emergency rope cutting and whatnot.

This concern is also almost non-existent, since sharks don't know what humans are underwater, and are curious/cautious of us, and we are too big to look like a direct threat or food (shark attacks on the beach happen because a shark is usually lost/desperate, and humans look like seals splashing around underwater). There's a reason why shark diving is an openly advertised thing, no extra steps or protection needed.

Hell, since sharks get paralyzed by swimming vertically, it's actually easy to deal with them if one gets frisky, such as here:
https://www.youtube.com/watch?v=UQSP8Tn-mdc&ab_channel=TCZwag
(Note, that video is obviously situational).