Hi Mike, it seems like your impeller boot is missing but I kind of agree with others this isn't likely to degrade performance to the degree you described.
It also seems you have a new or rebuilt replacement impeller recently, I can see the numbers written on it in marker. This probably means you've been experiencing some pump issues recently and had some work done already. We don't know those details but I wonder if some shop did this work for you, maybe that experience didn't go as well as anticipated?
Anyway, it's not difficult at all to replace the impeller boot, clean the contaminated grease from the hub it might have abrasive sand in the spline grease and should be filled with clean waterproof grease, the boot will help to keep the grease in and sand/dirt out of the grease.
Another piece that could be causing impeller cavitation/ventilation issues might be the carbon driveshaft seal, to replace that you must first unlock the drive shaft before removing the jet pump. unlocking the carbon seal retaining ring will allow you to remove the drive shaft after removing the jet pump.
Ans, the entire system needs to be checked, including the impeller torque on the impeller shaft, b/c it's common if the impeller wasn't properly torqued this could shove the drive shaft up into the engine crankshaft and damage the engine severely.
So if you intend on doing this work yourself, you MUST follow the factory service manual instructions exactly, to avoid making expensive mistakes!!! I think you can do this work and might need to buy some small simple tools but if the sequence isn't done according to the factory procedure I'm afraid if some important step is skipped you may suffer some expensive damage.
PLEASE download the factory service manual for 4-tec before diving in, read through the entire procedure for replacing the impeller and carbon drive shaft seal, and I think you should reverse whatever work was recently done. Unless the work was done by someone who knew what they were doing. Serious engine damage could result and it's $$$$ expensive to replace an engine.