The average depth of the Irish Sea is 60 meters, with a maximum depth of less than 300 meters. The location where the Obelisk fell was about a hundred meters from the surface of the sea.
For the average person, free diving to a depth of one hundred meters is an extremely difficult record to achieve. Without day and night professional training, the possibility of successfully challenging the record is slim to none.
Going down was possible, but the key was whether one could come back up. Many would simply sink to the bottom.
For Mages, free diving to a depth of one hundred meters wasn't a big problem; even without using magic, their strong physical condition ensured that they could easily make the round trip.
Four Mages pushed away the water current, creating a magic version of a submarine in midair, and descended to the vicinity of the Obelisk.