The cover is sealed to prevent dust from getting in, but when crossing water, the quality of the water will come into play. If you cross a muddy river, you will ingest the silt that is in the water, and it will shorten the life of the belt, as it will act as an abrasive inside the cover.
Not a problem if you flush afterwards, but it will depend on what you are crossing.
Kary

1987 Mitsubishi Pajero "Modified"
The other original Mitsu brother
Northwest Trailmaster