You will have service when you board however once you are 3 miles out to sea you will be into International waters and data roaming will kick in - this can prove very expensive so we recommend either switching to airplane mode or purchasing the cruise line internet plan. Click here for cruise line wifi pricing.
Most cell phone providers offer an international call/text plan - for example, AT&T runs $10 per day when you use it.
Most cruise lines offer a mobile app that you can use while onboard the ship, some offer robust texting and calling (to other phones onboard)for a minimal fee others simply offer onboard activities etc.
Ship-to-shore calls from your room are quite expensive, sometimes upwards of $9 per minute.