boatfly
/ˈbəʊtflʌɪ/Chiefly British. Now rare
noun
A predatory water boatman of the genus Notonecta (family Notonectidae), which swims on its back using its long hind legs as oars.
Origin
Late 17th century; earliest use found in Nehemiah Grew (a1641–1712), botanist and physician. From boat + fly.