One answer to this is that the lake actually no longer exists. The Somerset levels (between Wells and Glastonbury) are marshland that originally would have been flooded for large parts of the year. Over time, humans have drained the land for use, but one theory is that this is where the Lady of the Lake and her servants hung out.
Also, King Arthur isn't buried anywhere, he is sleeping in The Lady's care until England's direst need shall come when he will wake again.