Do you serve in a ship or on a ship?

Answer 1

Naval convention is you serve 'in' a ship.

An occasional exception - spoken or written - in where the ship's name mixes poorly with 'in'. Thus "...had two years in DIDO and then a short stint on ACTIVE before..." - the latter avoiding the implication of 'in-active' (ACTIVE and OPPORTUNE are two names that come to mind). Using in versus on isn't a hard-and-fast rule but to avoid be thought a landsman, when amongst (Naval) mariners 'in' is the better choice.

Linked to this, ships in the Navy are referred to as 'ships', never as 'boats' - the latter being slightly insulting. However, submarines are correctly referred to as 'boats' not ships (as to why, I don't know).

Answer 2

You serve aboard a ship.

Answer 3

You Serve IN a Royal Navy Warship or Shorebase but are, once up the Gangway, or throught the main gate, either onboard or aboard each one as you prefer.

The WW2 Movie about Lord Mountbatten's Warship HMS Kelly was called 'In Which We Serve'

A common error is also to write or say 'the HMS Whatever' It should either 'HMS Whatever' or 'The Whatever' with the capital T in 'The'. You would not write or say 'the Her Majesty's Ship Whatever'