Firstly the Watch would need the additional O2 fee to add a new number eSIM to the watch itself for use in the UK.
And yes unless O2 provide a cellular signal or allow roaming on the Watch as well as the iphone you will not get cellular service. You should ask O2 about that if you really want to use the watch without the iphone in your pocket or bag.
Most people would use wi-fi on board and possibly wi-fi calling unless the ship has cellular, and local cellular roaming, possibly on the iphone only, when ashore.
However ships usually have their own time zone "Ship's time" and you are recommended to carry your iPhone that has the ships time with you when ashore otherwise the without the phone close by Watch will revert to local time.
Personally I would not think it was worth the extra £10 a month just to make calls on the watch alone when I can do that anyway on the cheaper model with my phone in my pocket, which it always is!