If it's larger than 720cc, it will have two RAVE valves mounted on the cylinders, one each. With a black plastic cover over them about 3" diameter.
If it's the 720 and smaller, there are none. BTW, the 720 is a fantastic motor due to it's simplicity.
Yes I agree re:filter, the faster you desire to go the more fuel is required so a fuel filter (there are two small very fine ones inside carburetors (1 each) and there's one on fuel line called filter/water separator cup)
If the carburetors have never been rebuilt, you're getting close, that's a long time. Making sure to never have stale fuel in the system helps, even improves the engine lifetime.
But 1st check that fuel line filter (water separator), might find some junk in there. You can clean it using carburetor cleaner in an aerosol can (acetone) without damage (ie: I cleaned mine this way). Be careful to get the cup back on tight enough so there's no air leak b/c if there is a leak, the fuel pump on the carburetors will suck air and not much fuel resulting in fuel starvation. Some light 2-stroke oil or mineral oil on the threads and o-ring is fine and can help. Careful not to cross-thread.
If you overtighten the cup, it might crack.
Also suspect the fuel shutoff valve, sometimes they make a great place for junk that collects in the fuel tank to be caught and plug it up or the valve o-ring seals dry rot and you get air in the fuel instead of just fuel as the fuel pump on the carburetor pulls fuel from the tank air gets mixed in. Fairly easy to replace and not terribly expensive.
Lean-surging can cause detonation, which means the fuel stops burning prematurely (due to lack of adequate fuel), detonation is very hard on the pistons, ESPECIALLY a 2-stroke. Avoid lean surging conditions, resolve problem before continuing to ride.
If/when the carburetors need rebuilding make sure to have someone who knows the older Yamaha or Seadoo jetskis equipped with carburetors, the repair will go much smoother. Otherwise do this yourself "with a little help from your friends" if need be.
There are some small 3/32" inside diameter oil lines running from the oil pump to the oil injectors on the intake below the carburetors, these rubber lines will eventually rot and cause oil leaks and starvation of lubricant. At this age those are getting very old so you might want to replace them. I use 3/32"ID Tygon F-4040-A tubing, many lawnmower shops have this or ebay.
