one boat; one ship
Counter for boats and ships; used with native Japanese numbers or in counting contexts.
One fishing boat is anchored in the harbor.
Not a single ship passes by this island.
一隻 is a more general counter for ships and also used for large vessels, while 一艘 is specifically for boats and small ships.
一艇 is used for small boats, especially rowboats or racing shells, and is less common than 一艘.
Combination of 一 (one) and the counter 艘 for boats. The reading いっそう results from the assimilation of いち + そう.