Mastercraft Boats, as an incentive to purchase their product, offer a 5 year Warranty on their Upholstery.
Strongly recommend you make a Warranty Claim - quality of the Vinyl is obviously substandard and discolouration of this nature just should not occur.
It has happened to me (and to many others) and my Dealer accused me of letting "Bore Water" stains from our sprinkler system get on my upholstery.
The real truth is, the product is unsuitable and is incompatible with the Mastercraft supplied boat cover !
Other than the Upholstery sadly, my Mastercraft is the best on the river.