There is no definitive answer to your question as it all depends on what class of boat you serve on and what your tasking etc is, skimmers I can’t comment on but submarines you would work 6 on 6 off 6 on 6 off in defence watches whilst submerged or at sea, speaking from the warfare branch or most fwd branches, back aft do there own thing and depending on boat class SWS are in a complete different world of there own. Alongside we keep 24 hour duties usually 2/3 a week including weekends. But it all depends on your watchbill numbers. You need to know what branch your going or what class of boat. The best way to think about it is your joining a fighting arm you will work what ever hours is required from you wether it’s 18 hours a day on damage control exercises etc or 24 hours in wartime we are here to get the job done.