Receiving Ship (Wikipedia
has an unusually clear definition:)
A receiving ship is a ship that is used in harbor to house newly recruited sailors before they are assigned to a crew.
In the Royal Navy, the use of impressment to collect sailors resulted in the problem of preventing escape of the unwilling "recruits." The receiving ship was part of the solution; it was difficult to get off the ship without being detected, and in any case most sailors before the mid-19th century did not know how to swim.
Receiving ships were typically older vessels that could still be kept afloat, but were obsolete or no longer seaworthy. The practice was especially common in the age of wooden ships, since the old hulls would remain afloat for many years in relatively still waters after they had become too weak to withstand the rigors of the open ocean.
Receiving ships often served as floating hospitals as many were assigned in locations without shore-based station hospitals. Often the afloat surgeon would take up station on the receiving ship.http://oldweather.s3.amazonaws.com/ow3/final/USS%20Thetis/vol007of024/vol007_174_1.jpg
In this case, the Receiving Ship was the destination of crew being detached from Thetis, not new recruits.