Hi Alan,

I do the same thing. The ziploc bags I use, have a frosted area on which to
write on. They usually are the "freezer" type ziplocs. They have been in
water for quite a while, with no fading or smudging of the writing at all,
even after days of immersion. Just make sure the bags are dry before you
write on them, and let the marker dry for at least a minute before you put
them in a cooler or whatever. If you still have a problem, check the marker.
I always use the "Sharpie" type by Sanford, and I have never had a problem.

Scott
Florida