The first thing you will need is a walking foot sewing machine. Making cushions is not a beginner undertaking. I got serious help from a friend for how to pattern and get the tension/sizing close to correct. Thinner, meaning cheaper material, often gets loose spots and bunches up. A thicker material is the way to go. Make sure to use UV proof thread, like a polyester, and at least a #18 needle round tip.
New cushion material is often needed, along with new plywood seat bases.
This is just 50mil from Sailrite. There are a ton of marine fabrics available, but in the US Sailrite is one of the better quality suppliers for materials. If you poke around the website they have a ton of info on materials, and even awesome Youtube vids showing how to make lots of boat parts.
https://www.sailrite.com/Fabrics/Mar...ckness/50-mils