I believe Allan owns a Singer 107, which is more than up to sewing boat cushions. I also suspect Allan has Sunbrella or some ultrasueda like fabric to use. Even if he doesn't, velour blankets from K-Mart make decent boat cushion covers.
Making boat cushion covers for a Cal 25 is not all that difficult. Use the old covers as patterns, or use the foam as a pattern allowing about an an inch extra all around. Cut the top and bottom. Cut a long strip about two inches wide. Seam as below to make strip at least a few inches longer than completely around the edge of cushion. Buy a length of 1/16" or so cording (can be larger). Then cut the side piece, long enough for both ends and the front, about an inch up and down extra. Cut the back side piece, plus two inches for the zipper, plus an inch up and down.
Double the back side piece over, outside edges together. Stitch a loose row down the folded edge, in about an inch. slice the fold open and press down. Lay the zipper teeth on top the stitching, and sew each side of the zipped to the back piece. Rip out the stitches and you have the zipper installed.
Sew the back piece to the end of the side piece, laying the pieces together at a right angle, sewing across at a 45 degree angle. When you open the new side piece the seam will end up on the diagonal.
Wrap long, thin strip around cording and stitch the length. This is the piping.
Staple (using a desk stapler, or better yet a one-hand stapler) the back piece along one entire edge around the bottom piece, outside faces together. Work at it until you get it smooth. Then stitch the side to the bottom.
Then carefully staple the piping between the side and top, all around. Work until you get it smooth. Then stitch the side and top together. Stitch the ends of the side piece to each other.
Pull out staples (they rust).
Turn cover right side out. Wrap ends of cushion foam in dry cleaner's plastic wrap (to make the foam slide easily against the fabric) and insert.
Put on boat and go sailing.