Any elevated object on land which serves as a guide to mariners, such as a hill or steeple. (noun)
A beacon, buoy, etc. placed in the sea to aid navigation. (noun)
Examples of word sea mark
Onward they sailed along the south bank of the estuary, past the great sea-carved stone arches of “Île Percée” that made it an important seamark.
The men came back saying they'd set up a seamark, but there was no place for a port.
The ruins of the Cistercian Church which once graced this shore and raised above the trees its lighthouse tower, a seamark by day and a beacon by night, are among the loveliest in Wessex.
Where lies your landmark, seamark, or souls star?
He rubbed his glass, and called some other officers to verify the absence of the ancient seamark, but all they could make out was a white cloud, that might be smoke or dust or mist hanging over the town.