is the finish waterproof?

I am not an engineer but I will relay my experience. I make stick pens from wood (not Kits). My previous experiences with other finishes were all failures when I tried to carry them in my hot sweat soaked South Carolina pockets. I switched to a Starbond CA glue finish and it has held up 100% to all of my normal carrying. I have never tried to submerge them but in all of my normal carrying in wet nasty pockets all day the finish seems impervious to the elements. I did have one pen suffer some minimal deterioration when I lost it in the grass and recovered it a month later after it sat in the grass through a month of thunderstorms. Hope this helps. John

Does it give out white residue like most superglue do?

The white residue is a result of the glue drying too fast. This can happen when there are contaminants, dirt, oils, or excess moisture present on a surface. Therefore, it's advised that you properly clean the surface before applying the glue. Other times the white residue may appear is when too much glue is applied to a surface and the user wipes the glue off with thin tissue paper. In such case, the glue may scratch off tiny pieces of the tissue paper, leaving some of the tissue fibres on the surface. To conclude, this is a superglue and it can leave a white residue. But, if you consider the details explained above, you'll have no problem.

Will this hold the strap Into a rubber beach sandal

Superglue holds rubber tenaciously and is waterproof when dry. This will work better than rubber cement which would be the wrong product for such a repair.

what materials do you use this with, such as leather, wood, etc.

I use it mostly for working with wood. Primarily I use it, usually in combination with sawdust, to fill and seal cracks and voids. It also works well to harden a small piece of soft wood when turning things such a pens or hardening a small punky section of a larger piece. It will sometimes work to glue something together, but usually it is too thin for that purpose.