I can't imagine a satellite phone being cheaper than just getting the cruise ship package from Verizon (for voice) and/or the WiFi package from the ship ($89/GB) for data. IIRC, Verizon charges $2.49/minute to use the phone on board when not in port, but you can get it for much less if you get a cruise package.
Plus, a satellite phone requires line of sight between the phone and the satellite, so it's not going to work from your stateroom or anywhere else inside. Both the ship's cellular and WiFi systems should work from almost anywhere on board.
BTW, I know someone that went to the trouble of renting a satellite phone for a cruise. This was about 12 years ago, prior to every cruise ship being outfitted with cellular and WiFi service. They did have traditional "ship-to-shore" direct dial, just like the Disney ships still have from the stateroom phones, for about $7/minute. Even in that situation, it was a very poor experience for them. You can't use it to be reachable for inbound calls because of the line-of-sight requirement. And for outbound calls, how much are you really going to talk to overcome the equipment rental fees? These days, with relatively inexpensive cellular and WiFi data, it's a no-brainer: just use what the ship provides, to the extent you need it.