At risk of being a killjoy, in the event the captain cannot drive you to the event, the usual routine is as follows:
Six weeks & 8 bank holidays paid leave per year 'usually taken as 2 weeks Xmas, 2 Easter & 2 Summer. there is a degree of flexibility as to when they actually occur- ie: Christmas leave could well be given in November or January or even in March if you are deployed (at sea) over this period.
In addition, if your ship is alongside in UK or you are shore-based, you should anticipate working one weekend in four. If you are in UK and have to work a weekend (duty) then it's possible you may arrange to swap a duty with another individual so that you may attend a family function, or whatever.
If you are away from UK, you may be granted compassionate leave for a close family funeral, serious illness or injury, but routine stuff like Grannies 80th, your 21st or someone's wedding cannot be guaranteed.
If you have a friend who intends getting married, the best advice is to tell them simply not to bother. Instead: Find someone they really dislike intensely & then encourage them to give them all their money & children - it saves the hassle & reduces legal fees etc.