Movement by sea would have been possible with a boat similar to the North Ferriby boats discovered in the thirties in Yorkshire. These have been dated to around 1000 BC.

The ancient Eygptians also had "sewn plank" boats that date back to 2000 BC.

In America there are european style arrow heads (totally different to the Asian style) that date back to 8000 BC. Europeans must have had sea going boats way before Stonehenge was built.