The answer is "pretty much whatever they wanted". The only tattoo I have heard of that was common to many old-time sailors was to have the letters H O L D F A S T tattooed on the backs of their hands -- one letter on each knuckle.
Fairly overdone if you ask me. Especially when 95% of people who wear them this day in age have never even been in the Navy, let alone any branch of the military. Sort of tacky and disrespectful I'd...
I believe that tattoos originated after James Cooke first arrived on Tahiti. Tattooing was very popular in the South Pacific. Fletcher Christian and George Stewart of Mutiny on the Bounty fame both...
I believe that sailor's got tattoo's of turtles to signify that they had crossed the equator. I have heard that there were all sorts of "rites of passage" ceremonies around this. I am not 100% sure...