Roberts tricks Morton into revealing their secret deal to the whole crew, who realize what Roberts sacrificed to get them liberty. The crew's respect for Roberts return in an instant, and they secretly send one more letter to Admiralty in Roberts' name to get him the transfer to combat duty he always wanted.
And then Doc tells Roberts before he leaves, so that Roberts can know what the crew did for him. Roberts, who had been saying rough things about the crew, immediately regrets the things he said.
When the boatswain asks Doc for the letter about Roberts' death so he could post it for the men to read, Doc instead hands them Roberts' letter thanking the crew of the Reluctant as the bravest men he knew. "Post this. It's theirs."