Appreciate that, but rather surprised the boat actually anchors as it may need to rapidly move for many reasons.
Also, anchoring a boat means a long scope (ratio of depth to length of anchor chain/chain+rope) and the chain being pulled over the reef. If the tide turns, the boat will change direction and drag the anchor chain around.
As a wreck diver, I just find that a bit surprising that the boat would anchor and effectively cannot use the engine whilst divers are in the water. Obviously we all use different procedures in different locations.
Divers swim back to the boat. In a genuine emergency, I imagine they would cut the anchor rope if it couldn't be retrieved quickly.