undershore
/ˈʌndərʃɔr/Definitions
1. noun
the area of the seafloor exposed by the receding tide, or the part of a river or lake that is exposed by a fall in water level
“The undershore of the lake was dotted with rocks and aquatic plants.”
2. adverb
lying or situated beneath the surface of the water, especially of the sea or a river
“The ship’s propellers were designed to operate undershore to minimize drag.”