What is the ruling on a fishing boat captain working on a vessel whose voyage lasts a full month, which prevents him from attending Friday prayers in the mosque three or four consecutive times?

There is no objection to working in this job. You are not obligated to attend Jumu'ah (Friday prayer) as long as you are traveling on board the ship. If you return to your country and are neither ill nor excused, then the Quranic address applies to you: [O you who have believed, when [the adhan] is called for the prayer on the day of Jumu'ah, then proceed to the remembrance of Allah and leave trade.]