The short man on foot placed his thicker wool coat on the tall man's donkey, and borrowed the tall man's thiner coat to wear in the water. The thinner coat was swept away.

The case seems to be applying the following laws: The short man is a Shomer on the linen and is guilty and should pay -- but the giver was also doing work for the Shomer, as he watched the thick coat. That is called "Ba'alav Imo" (Shemos 23:13-14), in which case the Shomer is Patur because it is Shemirah b'Ba'alim (see Bava Metzia, beginning of 8th chapter)!

But in the end this was done without permission, so Rava turned out to be correct in making the short man pay for the thin linen coat.

All the best,

Reuven Weiner