“being charg'd, we will be still by land”
'Charg'd' here means 'attacked' or 'engaged by the enemy,' not 'charged with a crime.' Caesar (Octavius) is telling his troops: unless they are directly attacked, they will hold their position on land and not fight at sea.
- TLN 2662historical topical
“his best force / Is forth to man his galleys”
Octavius Caesar reasons that Antony has stripped his own land army to crew his warships ('man his galleys'), leaving the land force too weak to fight effectively. This is the decisive strategic blunder that Octavius is now exploiting in the battle following Actium.
historical The Battle of Actiumclassical Plutarch's Lives (North's Translation) “man his galleys”
To 'man' a ship or galley means to supply it with enough oarsmen and fighting men to operate it. A galley was a warship propelled mainly by rows of oars (not sails), and required large crews. Antony has drawn soldiers from his land army to fill those rowing benches.