I have a 12JF (an "E" with factory diverter nozzle). Grease the thrust bearing (front of housing, zerk on top) with the best waterproof marine super-duty grease you can get. I buy mine fron DuaneHTP (he's local to me). I reccomend greasing it every time you get it out of the water to make sure to force any water that got in the bearing out. Only takes a few pumps so don't overdoo it.
http://hi-techperformance.com/
At the rear of the bowl are 2 brass plugs. Pull them out and put GL-5 (gear lube) in one till it runs out the other. 85w-90 is OK, I happen to keep 85w-140 arround, works fine too.
If teh drive makes ANY NOISE (other than a possible "clanging at idle from the coupling being a tad loose on the splined shaft) CHECK THAT THRUST BEARING. There should be no vibrations, growls, squeals, nothing. A good condition jet drive is quiet. If the bearing fails completely, it lets the shaft slide foreward, then 2 things happen.
1.-- the impeller eats into the suction piece, ruining both parts
2.-- The shaft pushes all it's load on the CRANKSHAFT, destroying it's thrust bearing and the crank, possibly the block too.
You should be good to go. Enjoy.