One of the first things we do when we get on board [or at least definitely before sail away] is to set the phone up for the cruise. I often doing it while sitting having lunch; everyone in our party does it together and that helps us remember everything to do. I also bring a cheat sheet to help remember what to do. There are a range of settings you'll probably want to adjust, e.g. background app refresh and others. Once all of those are done, then I turn to the cellular data/wifi settings:
For preventing roaming charges: Go into the "cellular data" setting on your phone. Turn OFF "data roaming". Then turn off cellular data itself. These two settings being turned off means your phone will not use cellular data. If you only do this though it could still connect to the 'cellular at sea' network and send/receive voice calls or text messages. To avoid those, you need to be in "Airplane Mode" which turns off the cellular entirely. I don't do Airplane Mode alone, because it is easy accidentally turn it off and then your cellular is back on - which is why I turn off the data roaming and cellular data -- if I accidentally get the phone out of Airplane Mode, the phone still can't use cellular data until I turn that back on, which you can't really accidentally do. Yes, it could receive a call or text, but as soon as I get the first one I will realize i am out of airplane mode and turn it back on. And we have a rule to not answer any voice calls since we know receiving them is $$$ and means we turned airplane mode off.
For preventing any cellular being used turn on Airplane Mode. Then, you can turn back on JUST wi-fi so that you are able to connect to the ship's wi-fi.