At Black Diamond, they tested the strength of wet slings vs. dry.
“Dynex runners don’t seem to be affected when soaked, which was to be expected. Nylon runners, on the other hand, show about a 10% decrease in strength when wet. Should this slight decrease in strength be concerning? Not really. The strength of these, wet or dry, are well above the product rating and pretty much any force that can be created in the field. Bottom line: runners are crazy strong, wet or dry.”