This is strange advice. First of all, your phone already has a sim card (the way you worded it made it sound like your daughter suggested you put in a sim card, not replace the one that's already there). Secondly, many newer phones don't even have physical sim cards anymore, they're esims. Thirdly, a sim card (physical or electronic) has nothing to do with wifi. Sim cards are how your phone connects to the network so you can make basic calls and texts. As PP noted, putting in a new sim card but not activating it would avoid your phone trying to connect to your carrier's network and incurring charges; however, as PP also noted there are other easier, cheaper ways to do that.
The best way to avoid incurring international/roaming charges is to leave your phone in airplane mode for the entire cruise- that way it can't connect to any local (international) networks. The best way to avoid
DCL wifi charges is to just not sign up for the paid wifi-you'll have to connect to the free wifi to use the app, but there will be instructions on how to do that and there's no way to 'accidentally' connect to the paid wifi-it will clearly ask you and tell you what the charges would be.