is this bag waterproof?

All natural leather readily absorbs water, even raindrops. There are many things on the market to preserve or waterproof leather. Mink Oil (lanolin & silica) is a good preservative and is water resistant. For true waterproofing, get Sno Bol, (made from beeswax) it is perfect. Apply it to your leather shoes (and soles) and your feet will never feel wet again. Perfect for a dry day in work boots under the worst conditions. Be forewarned though, in the process it will definitely darken the leather. You should test it first if that bothers you. The effect is darker, richer, softer, a satiny sheen, an aged effect. The differences between imperfections and good leather will be accentuated. All that and weatherproof too, What more could you want? How to?: Turn your oven on to it's lowest setting (about 150 degrees), Cover everything leather with a good layer of sno bol, Turn oven off, Pop in the oven for about 15 minutes, (This will cause most or all beeswax to soak into the leather Rub off all excess wax and polish the leather. It should look that satiny sheen. If it feels too waxy, repeat the oven to polish steps again with no wax. This does not interfere with leathers nat.'al ability to 'breath'. You work shoe men will appreciate that.

How does this bag fasten?

There is no fastening. The flap is long enough so it is not necessary. I was very pleased with the quality and my nephew loves it.

Will this hold a laptop that is 15" wide? Asking as the bag itself is 15" wide so it seems that a laptop of that width may not fit.

NO! I have a Toshiba Satellite C55A and it does NOT fit. It measures exactly 15 inches across. You have to angle it and it bulges out at the sides and will not accommodate a sleeve.

should I get this bag for a 13.3 inch macbook or go for the 13 in one?

I'm not sure what the 13 in one is, so I can't help there. But I can say that I use my 15" MacBook in this messenger bag and it works well. Would a 13.3" MacBook work well with this bag? I'm sure it would obviously fit, but you'd have to watch that because it's a little smaller it might fall out of it more easily. There is a velcro "securer" for your lap top within the bag to help keep it from falling out, but I found that after a time it didn't close as securely as I would like. So there might be some risk over time. Hope this helps. I like the bag.