Just stumbled across this. Thought it might have a Liverpool interest.
Wow. Physical description. That would be good.You can now search 359,000 records of Merchant Navy Seamen for the period 1835-1857 on findmypast.co.uk (They already have 1918-1941)
The information the records hold about your ancestors can vary, but they usually include name, age, place of birth, physical description, ship names and dates of voyages.