The guard hairs of seals form a waterproof barrier due, in part, to their flattened shape.

References

"Guard hairs may be flattened in shape; this adaptation is most marked among aquatic mammals such as seals, so it is evidently a waterproofing device." (Foy and Oxford Scientific Films 1982:83)

Book
The grand design: Form and colour in animalsBLA Publishing Limited for J.M.Dent & Sons Ltd, Aldine House, LondonJanuary 1, 1970
Sally Foy