I have noticed that Sea-doos (or at least, some makes of Sea-Doo equivalents) have a vertical plume of water coming up from behind the driver’s seat - the height of the plume varies with the speed of the Sea-doo.
What is the purpose of this plume? I can speculate that it’s the emission from a pressure release valve, put in the works to ensure that the engine doesn’t overload the drive-pipe (or whatever you call it), which might otherwise damage the engine … but what’s the Straight Dope?
I don’t know what its purpose is, but I can tell you that it’s called a “Roostertail”. At least, that’s what everyone I’ve ever met called it.
(I hope this doesn’t qualify as a drive-by post)
At least on the old Yamahas, it is just a takeoff from the impeller’s output. It don’t do nuthin but look good.
I thought it was the overflow from the cooling water, like on an outboard. If that stops squirting, your water pump is having problems and you gots ta shut off the engine. I just figured they decided to make a neat gimmick out of it by pointing it up instead of down.
I think that was a “feature” on Yamahas. I’m not aware that anyone else used it. I think the idea was to make the vehicle more visible to other boaters, so as to avoid collisions.
It could also cure constipation. 
BTW:
Sea-Doo = Bombardier
JetSki = Kawasaki
Wave Runner = Yamaha
What are Polaris called? Just “Polaris”?
I have a Yamaha GP1200…current Yamahas still have the plume. It IS a feature, and it comes in very handy here in Florida. When we go out riding in the jettys, the waves are usually higher than the tops of the driver’s heads…however we can see other jetskiers by the plume of water(if they are on Yamahas).
Abel–the roostertail is the water thrown up out of the wake, this is a small stream directed nearly straight up at the rear of the jetski.
It also helps if we go running up and down narrow rivers/canals feeding the intercoastal, as you can see the plume above the marshgrass around curves before the actual jetski comes into view.
I’m not sure why all manufacturers don’t do it, it can’t be very expensive, and it is definately handy.
Thanks, all, especially JaxBeachBoy! I must admit to some chagrin that my elegant “pressure release valve” theory is a lot of hooey.
Basically just for fun, eh, with some visibility-for-safety thrown in? Personally, I’d prefer a pennant on an aerial - or even better, a thick chain constraining my movement to within ten feet of the bar on the beach!
I’d say its primary purpose is for visibility.
As far as a pennant, there may be some small problems with that. I’ve always boarded JetSkis from the rear when I’m in deep water. Everyone I know does the same. A flagpole would make that difficult.
Bad idea chaining it to the beach. Where’s the fun in that? 
I really wish all PWC manufacturers did something like it…to say that it is disconcerting to jump a 6 foot wave only to find another PWC coming under you is an understatement.
I really wish all PWC manufacturers did something like it…to say that it is disconcerting to jump a 6 foot wave only to find another PWC coming under you is an understatement.