Re: Recovering Seat ?
I haven't done any boat seats but I may have done a few chairs. The math is simple. Assume a more or less square seat for this example.
The surface area of the seat plus the the rise (edges) on all four sides plus 6" ( maybe just 4" if your grip is still good) more on all 4 sides, some of which will be trimmed away after stapling. A 16x16 seat with a 4" rise all around would take a piece 16+4+4+12 or 36x36. One yard per seat assuming there is no pattern match (repeat) to contend with.
Haven't done vinyl but with dinning chairs leaving the old fabric on (1 layer only) makes it far easier for me to get a good stretch.