Relationships at sea are tough. You say that you understand his job, but I challenge that’s not entirely accurate. Every sector of the industry and every vessel is different.
On a working ship, almost every spare moment of time is spent trying to catch up on the lack of sleep. There’s hardly any real downtime. You want him to put in the “minimum effort,” but anytime he’s talking to you that IS actually a substantial effort because he’s sacrificing his very limited time. Wi-Fi connectivity, time zones, ship’s schedule/operations all pose barriers to communications.
When he doesn’t message you, it’s not because he’s unhappy with the relationship. He’s just busy and exhausted and doesn’t want to talk about the ship and his trip. Which leads to a point: when you do talk, I’d advise not to ask too much about work- a sailor mostly just wants to hear that you miss him, hope he’s doing well, and that you’re excited for all of the fun plans you’re making together when he gets home.
Just my opinion from his perspective and multiple relationships at sea. It takes the right kind of partnership and independence, but it can work. Only you guys can make that happen though, so I wish you the best.