Where the International Convention on Salvage applies, does the master need the power of agency of necessity to make a salvage agreement without notifying the property owners?

No.

Article 6, paragraph 2 of the International Convention on Salvage provides that the master shall have the authority to conclude contracts for salvage operations on behalf of the owner of the vessel. The master or the owner of the vessel shall have the authority to conclude such contracts on behalf of the owner of the property on board the vessel.