Take a look and report engine RPM, if the egine is foing near 7,000 RPM while the ski does much less than 50MPH there's something wrong with the jet pump.
I've seen 50MPH in my similarly powered boat under optimum conditions. Like most 2-strokes, if the DI isn't running correctly, it's likely to be damaged soon, if not already.
Lots of things to cause running issues both external and internal to the engine, cylinder compression of engine and condition of the jet pumps are the places to begin.
When buying any watercraft sold as turn-key, a water test of at least 30 minutes is mandatory b/c running on the trailer doesn't tell you much about how well it runs, only if it runs.
RAVE valves will need to be disassembled and cleaned, in many cases at a minimum unless this was already done recently. Mine had never been cleaned, and there were a couple other issues holding it back and might actually have saved it from abusive treatment. The compression was great but it didn't run out like it should but I bought it anyway with a good idea of why it wasn't making good power.
Another common issue on a used DI would be the electric fuel pumps are corroded and tired, those little guys work their butts off and dirty nasty stinky stale fuel is bad news in any marine engine, especially a DI. Maybe they sat around in the hot sun with partial tanks of stale fuel in their bellies for a few years and someone fired them up sucking nasty the muck through the electric fuel injection system?
A poorly running DI can be a steep a wall to climb for any newby and most shops these days have no choice but to jamb it to you with shop rates being what they are. Best to find someone who works on them everyday, if there are six or eight 2-stroke Seadoos in the service area and not many boats, that could be a good indicator the guy concentrates on them or at least knows what to look for, many times I think the customer pays a high price for mechanics who just aren't familiar enough with the particular model to make good judgments or don't have the necessary specialized tools and diagnostic equipment.
You can download the full array of Seadoo factory service manuals from the interwebs for various models, these are worth their weight in gold and I highly encourage you to do this, read them carefully cover to cover.