You need to take with you the kit relevent to the job you are doing and where your ship is going. You should take a few changes of civvies, but not too much as the stowage space is limited.
As people have said it depends where you are joining and where the ship is. ie if the ship is away then you need the kit for the climate you are in. If it's alongside in Pompey and not sailing in the near future, you will be able to go home and pick up extra stuff.