If the ship provides free wifi and internet, then iMessage and FaceTime would be free. Those services are always free from Apple, and you only pay something if you are charged for the internet connection you are using.
BTW, no cruise ship I have been on has ever provided free internet, since their internet is usually provided via a satellite feed, which is expensive (they may provide free wifi access to the ships intranet, but internet access is for-fee). Also, satellite internet is always somewhat problematic with timeouts and dropped packets, so FaceTime and any VOIP service as well is likely to be poor (and the ship may exclude such usage as well, if their satellite feed simply cannot handle the traffic).
Many ships now also offer some form of onboard cellular service, but once more than a few miles offshore, that too goes through a satellite relay so is usually an expensive for-fee package service.