Ancestry.com - Ship's Medical Journals

There are two collections available and both contain indexed images of the original journals. A variety of subscription options are available to access these images.
UK Royal Navy Medical Journals, 1817-1857 (ADM 101). Ships' medical officers were required to keep a record of patients, treatments and outcomes during a voyage. The 671 journals record patient details but they can also include details of other passengers, crew and day to day happenings on board. Most of the journals are for convict ship voyages to Australia but there are 29 journals for emigrant ships 1825-1853.
UK Surgeon Superintendents' Journals of Convict Ships, 1858-1867 (MT32). There are 11 journals for convict ship voyages to Western Australia 1858-1867. The information varies from journal to journal but can include name, age, crime, sentence, point of embarkation, native country, education, death date, cause of death, sick lists, lists of ship's stores, and day-to-day happenings aboard ship. Crew members may also be mentioned.
Note: You will find an index to the ships from both these series on Grahame Thom's site.