Super old paper, but I've always noticed how the few videos on the subject sit the fence on whether they were semi- or fully aquatic and how there are very few illustrations of desmostylians and how many even depict them as semiaquatic. This paper argues they were "essentially or exclusively aquatic", and it has multiple illustrations of different, proposed modes of life.
Various restorations of desmostylians based on morphological data illustrating the diverse lifestyles proposed. A, semiaquatic (like the polar bear) (from [17]). B, bottom wader (from Inuzuka's restoration; the figure is printed with the permission of the Geological Museum, Geological Survey of Japan). C, bottom walker, Hippopotamus-like (from [73]); D, bottom swimmer, sirenian-like (from [22]). E-F, active swimmer, pinniped-like