Why sailors, not Royal Marines, dominated the Altmark boarding

Were Royal Marines the main force in the Altmark boarding party?

No; destroyers carried few Marines, and even the cruiser-supplied parties included many sailors.

Destroyers did not normally carry the sizeable Royal Marine detachment found in a cruiser. Although personnel came across from the cruiser force, the available boarding parties were still dominated by sailors, whose shipboard duties and training already included this kind of close action.