Ok, a couple of things. First, he will be in the PRK, so that double-edged SOG would land him in the pokie before he even got off of the beach. That is LA/Orange county, on the beach. Heart of the extreme Left. They will be screaming for the cops about a big, bad man with a sword as soon as they can get their cellphones out of their pink, fuzzy purses. The girls might scream even louder...
Second, Great Whites... Yeah. A knife will help with that... If he is extremely unlucky, and gets attacked, it will hit him from underneth the kayak, and knock it, and him out of the water. And possibly take a bite to see if he is a seal, like he looked like from below. When it finds out that he isn't a seal, it will probably just swim away, grumbling that the seals in this area sure taste funny today. Of course if it bit him, he will bleed to death in a matter of minutes. No knife would do him any good in the world. We have had 3 great white attacks here in southern California since the 1950's. All 3 were fatal. One may or may not have actually happened, the other two were people mistaken for seals, and then spit out. The trick is trying to not look like a seal.
Finally, about which knife to take. If you think you will need a SAK for all of the tools, by all means take one. It probably won't rust in a few days. Try to keep it out of the water, and try to wash it when you get home. No biggy. And if it does rust, well, that is when you either clean it up, or go get a new one. Knives are meant to be used, not kept pristine and untouched. If you want a bigger knife, either use one that you don't mind getting dirty, or go to wallyworld and get a cheapy one that you don't mind thrashing. You will not find that many things that need to be cut between the beach and catalina. A serrated knife or at least one with some serrations might be helpful in case you run into some fishing line or rope. But again, you will be out in the water. There isn't much there to cut. Kelp is a non-issue. You can tear through it with your hands. The worst thing that it will do is foul your rudder, if you are using a sea kayak that has one. Just pull the rudder up while you are in the kelp. Problem solved. If you really, really, really want to take a bigger knife with you, you can always check the local dive stores, or online for a cheap dive knife, and figure out how to rig it to your lifejacket. But I would also advise that you do that BEFORE you go on your trip, so you can find out if it interferes with the way you paddle, or do anything else. Of course your instructor and classmates may laugh at you if you insist on bringing a Rambo special along...
-Mb