Having had a similar hole in a backpack once, I found that transparent repair patches for air mattresses work best, and are hardly visible in the end. They are waterproof and self-adhesive. Cut out two patches of the same size, ideally in a round shape or at least with rounded edges to avoid tearing them off by accident. Draw the edges of the hole together a bit and fasten the first patch from the inside of the bag. Then turn the bag around and fasten the second patch from the outside. Like this, the hole is completely waterproof once again, and the edges are protected from tearing any further.