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 say.
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...
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...