How to repair leaking boat canvas?

Have to agree with "Beeswax" We used this 55 years ago when I first went to sea and my first bosun had been using it for some 50 years before that. It is a good natural product, no chemicals involved so won't hurt any thing. It is reasonably priced and even cheaper if you know some one who keeps bees. When forming seams we would rub the canvas with it, his protects the inside of the seam, it also heps to hold the fold in place as you stitch the canvas. if you have to restitch any of the canvas give the sail twine a good coating as well.

the beeswax works well so does silicone spray but if the canvas is looking very old & stained you might like to try giving it a coat of a water base exterior house paint. Clean & degrease it thoroughly first, apply one good coat & let it dry thoroughly & dont fold it up for at least a week. It takes that long for acrylic paints to fully harden

Try using tent seam sealer, available at Bass Pro, Cabellas, etc.

Goop glue might work, but I recommend a new canvas.