Sorry I was absent on this... but I'm not really a 4stroke guy. (i'm cheap. LOL)
Anyway... it looks like the last owner liked to run shallow, and beat that pump up. BUT, I've seen way worse. TO be honest... it's not all that bad. The wear ring has some wear... but I don't think it would cause the cavitation you are describing. But lets start at the beginning.
The first thing I would do, is take a small grinding tool, and clean up the stator veins. A die grinder is good... or a Dremel. If you don't have one... there are import versions for less than $20 and they come in handy for things like this. Put a pointed grinding tool on it, and clean the burs off the stator. Then, put the sanding drum on it, and make sure there are no sharp edges on the plastic veins in the mount. And finally... carefully clean the sharp points of the face of the impeller. You don't have to try to grind away the face to the point where the divets are gone (it would wind up being out of balance)... just take off the jagged edge. If there is a bent spot... try to support it, and flatten it back out with a hammer.
Now.... the kind of caviation you are describing can only be casue by 2 things. One... an air leak, and Two... blocked pump.
Lets start with the blocked pump. If you suck up a plastic shopping bag... a chunk or rope... or even a hunk of wood, and it gets jammed into the pump. Those things will keep you from going. BUT... there's no moving around to get you on plane. Also... those things will cause horrid vibration. It's normally obvious when this happens. You will be running just fine... and next time you throttle up... you get vibration, and little thrust.
The other side is the air leak. If you've ever had a hole in a drinking straw... you will know exactly what I'm talking about. If not... next time you are at McDonalds... poke a hole in your drink straw, and try to take a drink. (you will get almost nothing even on a small hole) Same thing happens on a jet drive. That air leak can happen around the shoe, around the pump, or most likely... the carbon seal. Since your pump has been beat up some... take a good long look at the plastic shoe in the hull. We have to verify air isn't bypassing the inlet. Also... when you put that pump back on the mount... make sure the seal is in place. If yours has the neoprene seal... replace it. If you have the rubber seal... make sure it's not hard and deformed.
Now for the final (major) issue... the carbon seal. It's a good design for safety, but has some of it's own issues. The Seadoo driveshaft seal is a very simple carbon seal system. There is a fixed stainless ring on the driveshaft. There is a carbon ring that rides on that stainless ring... and a rubber, corrugated boot that acts as a spring to push the carbon seal into the stainless ring. The lake water comes into the boot, and up the face of the seal. It cools, and lubes the seal. (that's why it is impossible to have a Seadoo without water in the bilge. It leaks by design) this is also why it's bad to run a Seadoo on the trailer. (kills the carbon seal)
This seal is very safe, in the fact that is has almost no chance to have a catastrophic failure. it simply wears, and starts to have leaking issues. It also has zero maintenance. In most driveshaft seals... there are bearings, and rubber seals. These "Stuffing Boxes" need to be checked, and greased after every few outings. The issue is... if you don't take care of it... water gets in the bearings... rusting them... and when it rips loose... it leaves a rather large hole in the back of the boat... and then it sinks.
So... at this point, I'm going to assume you have a 12 year old seal. It's time to replace it. The wear comes on the face... and that drops the pressure from the boot. It comes on the inner edge... and makes a bypass point (thin spot where air can come in) and the boot itself will get stiff, and lose tension. Oh... and the stainless ring has a couple O-rings inside. Those can split, and allow air to come in. So... order a new carbon ring, O-rings, and boot.
If you want to verify this is the issue... have a helper pull the boot back, and put a blob of waterproof grease on the face of the carbon seal. Then, put a fat zip tie in one of the valleys of the boot. (it will add tension and keep it from getting sucked back) DO NOT use anything hard to put the grease in, or pry back the seal. It is a hunk of soft carbon. You can crack it easily.
Hope that helps.
Oh... and for a question you posted above... a Skat-Trak "Swirl" impeller will always give you the best out-of-the-hole performance. (solas is normally better for top speed)