While this may be an older thread, since Google keeps bringing it up in searches, it's probably worth correcting some misunderstandings.
Plastic boxes (a.k.a. Corplas) are more water resistant that corrugated, however, there are some significant "Yes, buts" in that answer. The operative word is resistant. It is true water will not penetrate the material, whereas corrugated will absorb or wick water over time. But all corplas boxes are made of a flat sheet and use a fold over design to form a box. This means there are open seams on every corner that water will readily leak through. If you worry that water may come from above, such as from a broken pipe, then they offer some limited protection. But they offer no protection from water from flooding.
Another characteristic of corplas is it is less rigid that corrugated. This means the material is subject to flexing and will not support much weight. If you anticipate stacking boxes at all corplas would be a poor choice. One of the advantages of corrugated is it can be made with different weights of papers and fluting to increase strength, both stacking and tear resistance. If you had problems with corrugated tearing, it is likely a cheap grade of cardboard. Avoid any boxes that use 200# or 250# test cardboard. That is a low grade which, unfortunately, is used by most comic box manufacturers.
Another common misunderstanding mentioned in this thread is the need for acid free. The source of most acidity that yellows comic pages comes from the lignin in the newsprint. Since acidity migrates from more acidic to base, typically the boxes are more in danger from acidity than the comics themselves. Where this becomes really important is in your choice of backing boards since they are next to the comics. Most, including "acid free coated", can be a source of problems. There is a study about this posted in the preservation blog at www.CollectionDrawer.com.