My new watch is completely waterproof, so I can swim with it.

waterproof 是由 water（水）和 proof（防...的）组成的复合词。proof 作为后缀时表示"防...的、抗...的"，所以 waterproof 字面意思是"防水的"，指物品能够防止水分渗透或损坏。