$9000 for a 2004 N/A (155hp) seems a bit high to me for a 2004. I bought my 2004 almost 3 seasons ago for $6300, with everything except a boat cover. It was in really nice shape, clean, ran well etc. Got it in SW Michigan. Unfortunately for me, I was a novice with these jet boats and soon after purchase ended up rebuilding the propulsion system. Even so, after everything I spent I still feel I got a fair deal. Boat runs like a champ and have never had any problems since.
I purchased from a private seller, so I imagine there is a retail markup at a dealership. That said, if everything is perfect with the boat I probably would not spend much more than $7,500-$8,000. The real challenge will be to make sure the engine and propulsion system is in good shape, and you really cannot do that without a water test as others have commented.
If you do plan to pay top dollar for this boat, you might consider taking it to a qualified Seadoo shop and have them look it over (unless you have hands on experience with these boats). They may be slow this time of year and can go over the boat for a reasonable price, plus they would have the computer system to check for any trouble codes and the exact hours on the engine. (not sure if the dealership you are buying from is a Seadoo shop or not) Also, you mentioned this is a brokered boat so the original owner may still be in the picture. It is possible you could ask for any past service or maintenance records.
Whatever you do, the 155hp 4-tec engines are pretty rock solid if maintained properly. As long as the oil, coolant and spark plugs were changed regularly; and the jet pump was serviced when needed then it should be a good boat. Make sure all the electrical stuff works (blower, bilge, radio, instruments, fuel gauge, etc.) I hope everything works out and I think you'll be happy with a newer Sportster. I have this boat, so if you have any other questions about them just let me know.
--Deven