How waterproof are these? If I step in a puddle will it soak up water like a sponge?

I've stepped in water no problem. Tops are water resistant. Nice and warm.

my mens size is 8.5. 8 will be very small? I think they will stretch out more. What do you think?

These will not stretch. You should size up to a size 9 if you’re regularly an 8.5.

Can I wear these to a football game?

Absolutely… the tread is excellent for indoor and outdoor. The fit, comfort and the “north face” insulation will have you stylin and profiling while warm and comfy….. Game on.. Enjoy and impress.

Do these compress relatively flat?

I don’t know how much they compress because I have not traveled with them. I do know they are light weight, warm and comfortable. I’m sure they will compress but I don’t know what impact it would have on their shape long term. Hope this helps.