I used to use these in the boy scouts all the time. They're good knives for a boy scout on dry land, but a terrible terrible knife for a diver.
They will keep for a while... but won't last nearly as long as a good dive knife taken care of properly. There's too many nooks and crannies for sand and bits and organic material to get lodged into. Merely rinsing won't cut it, you'd have to open it up and REALLY clean it after each dive trip. They will eventually rust, and even if not rusted, they will become harder to open due to the pivot points/hinge deterioration due to repeated water exposure.
As mentioned before, if you have gloves, it's possible to open one, but damned hard.
If not used carefully, these knives have a tendancy to snap shut on your hand. I've seen many a Tenderfoot with a nice bloody gash on one of their fingers. I've had my share as well.

These knives don't stay very sharp if you use them a lot. You'll need to maintain the edge a lot more than you will with a traditional dive knife.
Lastly, and I believe most importantly, in an emergency, you're not going to be able to employ your swiss army knife nearly as fast as a traditional dive knife. What if one of your hands is fouled? how the hell will you open your knife, even if you an retrieve it? It can be done, but not easily, and if you're in an air situation, you could be in real trouble. Plus, there are scenarios in which you might need your knife RIGHT AWAY. Not very frequently, but I have heard of them. The time it would take to unhook/pull out your swiss army knife, open it, and then employ it would be waaaaay longer than the time to pull a traditional knife out of it's sheath and free yourself.
If you really like the versitility of the swiss army knife underwater, bring it along. but carry a traditional knife too. You can get a cheap one, and it'll be worth the few bucks when you need it.