You can try to do the seat cover replacement on your own. Most of the older jetskies with one peace cover require only few simple steps:
1. Remove the old cover, you have to get a staple remover or struggle with a screwdriver or other tool that would do the job. Staple remover is definitely a must if you plan on doing more than one replacement and it is pretty cheap $3-15 on ebay.
2. Spray the old cover and try to stretch it on the flat surface. Heat gun or an iron(not very hot) would be useful. Heat up from the inside, since vinyl surface tend to melt from heat.
3. Once the cover is stretched, copy a pattern either on the thick paper, plastic or straight on the new piece of vinyl. Make sure that you are using marine vinyl and not the cheaper automotive or home furniture type. Marine will last you from 5 to 15 years depending on how well you take care of it.
4. Cut out a new vinyl along the pattern.
5. Lay down the new vinyl on the seat.
a) If the foam on the seat is in a good shape, just go to next step.
b) otherwise if the vinyl is cracked with some smaller holes, use a sheet of thin foam 1/2 to 1/4" thick and cut an approximate shape of the seat and using foam adhesive, glue it on the top.
When buying foam, try to find all weather foam that will last you a lot longer and is harder, so it will better cover your seat foam imperfections.
c) if the seat foam is completely deteriorated or missing, try to get the best approximation of the shape and foam stores usually would be able to cut the foam in that shape. Glue that new seat foam and continue to next step.
6. After laying down the foam, make sure it lies as well covering the seat from the sides and front/back, staple it with a couple staples on the front (dont put too many since you would be most likely removing them later to adjust), then stretch it to the back, use heat gun to warm it up for better flexibility and stretching and fix it with another couple staples.
Repeat the same with the middle left and middle right. Watch for the compression in your stapler, if you using a compressor. If staples punch through vinyl, reduce compression. Then starting from the back start stretching vinyl from the back, 3-5 staples on the left, 3-5 staples on the right. And make sure you are stretching well for each staple. Place staples and 2-3" distance.
When you get to the middle, you might remove original staples if they are creating kinks and continue all the way to the front until it is all done.
