Leather tear on Alpina Alaska stitch

[Leo Tasker]

I wouldn't worry about the waterproofing as far as your foot is concerned - the waterproof membrane inside the boots will be under the leather and foam padding, closer to the inside. The rip is also high up and on the back, so the chance of water/wet snow getting in is lessened, especially if you use gaiters or your ski pants have some built in.
If you don't want to go to a cobbler, I would get a decent sized curved sewing needle and use some dental floss to stitch the rip closed. You could go through the black fabric above and below the red leather strip, this will probably give a stronger repair. Then use a decent amount of shoe goo over the top to seal it and stop the stitching being abraded. Should be a decent lasting repair, just be careful about putting too much force on the grey pull tab when putting them on!
Cheers, Leo