Whether the mission was secret or not doesn't matter. This Joe Schmoe, a captain, confirms that the men were sailors and that they would fight their way in to deliver provisions. You guys act as if it were impossible that the men were sailors and that they were to fight if necessary. This letter proves you wrong because this captain fully accepted their status as fighting sailors.
This Captain Schmoe had seen the Powhatan and how she was outfitted?
They were to rendezvous prior to reinforcement of the fort. Powhatan was sent to Florida. It could have been manned by history's first transvestite army for all he knew.