Q: How do I share my Macbook Pro(2015) wifi connection with my Iphone 6?

Why can't you connect two devices to your network?

Open the Sharing preference panel. One of the options is Internet Sharing. Click on that to see what your options are. You will note that you can only share your computer's Ethernet connection via Wi-Fi for your phone. If your computer has a Wi-Fi connection then you could only share it via BlueTooth PAN which is a personal area network. I do not know if that can provide Internet access to your phone like Wi-Fi.

If you have cell service while on the cruise, then you do not need to use your computer's Wi-Fi since the phone would work independently. If you do have cellular service onboard the ship, then you could use your phone to provide a hot spot if your provider supports it and if it's on your plan. See Get help with Personal Hotspot in iOS, Continuity, Handoff, Instant Hotspot, and iOS- Understanding Personal Hotspot.