Re: reupholstering boat seats?
It looks like you are re-covering an existing seat. . . The trick is to get the new covering to fit tight and match the shape of the old covering as much as possible. You will want to use stainless steel staples to fasten it to the backing material.
I re-covered the rear seat of my Formula 242 back about 7 years ago (pic below). I used a household sewing machine with an upholstery needle. One of the tricks was to use the old covering pieces as the pattern for the new pieces and get things to fit as tight as possible. I also tracked down the same type of vinyl that Formula originally used (Nautolex).
BEFORE . . . .
AFTER . . . .
I changed the sunpad portion a bit, giving it the colored stripes instead of plain white of the original.
The only problems that I had was that the gray did not quite match the original . . . The red was an exact match. I re-used the black pieces from around the cup holders. Sewing was difficult and I used lots of clips to hold the pieces together as I was sewing. The small pads forming the back rest were fairly difficult to make, since they were so small.
If you can get the new covering to be as close in shape to the original, then the fastening will be easier and the end result will look better. I cannot quite tell how tight the covering is going to be in the pictures that you posted, but I would try to pull things taut and see what it looks like. It is always easier to tighten up the pattern by taking the seams in a bit.