If it was fast idling on either muffs or a tub of water ,I wouldnt worry about it at the moment, if it is still fast sitting on the water, check the throttle arm/s are touching the stop, and that the throttle cable not adjusted to tight and hasnt slipped, if all those are correct then i would advice to to contact your nearest dealer.
It should be idling at approx 700-800 in water, out of water on muffs will increase 100rpm.
If left at 1500 you are doing damage to the engagement dog when shifting into gear.
Is it actually revving too fast or is the tacho incorrectly set?
If it is revving too fast and cables are all adjusted correctly then the idle screw is normally wound in to mask stalling due to slight restrictions in the low speed jets.
