Pardon the suggestion of a lowly slave boy -
There is about a 75 mile stretch of straight river from Pelargir northwards toward where the Gondorian fleet is coming from (Atlas of Middle-earth - Fonstad). Wouldn't the brilliant captain of the corsairs have lookouts on the highest masts?
And such a lookout could send out a retreat call - horns, drums, etc - saying the fleet is in sight.
That way the corsairs would have a head start.
By the way - how many ships are we looking at for the Gondorian Navy? and how many for the Corsairs?