Modern riflemen in a Trafalgar counterfactual

What could a modern SAS regiment do if distributed among the British ships at Trafalgar?

In this counterfactual, riflemen in the fighting tops could engage exposed officers and musketeers before the sailing fleets closed to effective cannon range.

The proposed use is to place riflemen in the ships' fighting tops and engage people exposed on the opposing decks. Their range could allow them to shoot at officers and musketeers before the fleets closed for the usual cannon exchange. In the most extreme version of the scenario, British ships would find very few defenders above deck when they reached boarding distance. This is a deliberately unequal technology counterfactual, with modern weapons inserted into an 1805 fleet action.