If Queen Elizabeth's crew had spotted the frogmen

What if Queen Elizabeth's crew had spotted the frogmen before they placed explosives?

The crew would have thrown grenades into the water, potentially interrupting the attack.

The crew would have thrown grenades into the water around the divers, following the standard British response to enemy swimmers alongside a ship. The counterfactual assumes the attack could be interrupted before the explosives were placed. Preventing the damage would leave Somerville with more battleships available alongside Warspite. That benefit depends on the defensive action succeeding.