counter
counter for small boats
Used for counting small watercraft such as rowboats, canoes, and small fishing boats. Not typically used for large ships.
湖に小舟が三艘浮かんでいる。
Three small boats are floating on the lake.
漁師は一艘の船で沖に出た。
The fisherman went offshore in one boat.
隻 is a general counter for ships, including large vessels, while 艘 is specifically for small boats.
The kanji 艘 is composed of 舟 (boat) and 叟 (old man), but the historical derivation is uncertain. It is conventionally used as a counter for small boats.