It probably wasn't underwater in the mesolithic, I guess it depends how far from the shore they were. The headlands around Tynemouth have supposedly eroded a considerable distance. So possibly this was originally a land based set of objects that ended up in the sea.

How would you know they hadn't been dug up on land then deposited in the sea at a later date? People have been tipping (what they consider to be) rubbish off the pier for years.