In the Crystal Cave in the sea, a lake can be found. This lake housed some Inferi that were controlled by Lord Voldemort.

Tom Riddle

At some times when the orphanage known as Wool's Orphanage were visiting the cave Tom Riddle, which was Voldemort's first name, took some children near this lake and tortured them.

Voldemort and Regulus Black

Later on Voldemort ordered Kreacher out to the small island in the lake so he could drink the potion that was held in a little sink-like carrier so Voldemort could place the Horcrux locket and after that fill the carrier with potion again.

Regulus Black and Kreacher

Some years later Regulus sailed over this lake again to call Kreacher the House-elf out to the island and ordered him to drink the potion in order to let Regulus steal the locket and replace it by another locket and refill the carrier with potion. It is unknown whether Regulus Blacks body still lies there.

Harry Potter and Albus Dumbledore

In 1997 Albus Dumbledore and Harry Potter sailed over this lake to get to the island so that Dumbledore could drink the potion and let Harry take the locket (which was, unknown to them, the false locket). When Harry took the locket hundreds of Inferi came out of the lake and tried to drown Harry in the lake until Dumbledore made a Firestorm. When they had to sail away from the island Dumbledore made a passage in the storm to let them sail across the lake.