Is there Coverage over the Atlantic Ocean

I suspect there will be 2 types of coverage:

1.  Cruise ship roaming (very expensive typically) - and you may wish to disable your cellular services BEFORE boarding the ship - as roaming may cost you for received calls (even missed) and text as well as any data (background apps).
2. WiFi - While not ‘really’ cheap, it is much less expensive to obtain a WiFi pass - and it will be most likely a flat rate vs. pay per use.

For your cruise:

https://www.t-mobile.com/support/coverage/international-roaming-checklist#heading3