Should I be worried about my boyfriend joining the merchant navy?

We are both 21 and have been together for 4.5years, we do not live together as we are both students. Now my boyfriend is joining the merchant navy. We live in the north of Scotland so he will be away from home a lot but I was happy for him in joining and thought we would be ok. However, on telling friends and family some are putting doubt in my head that we'll last, and saying things like "oh you sure your happy with that?" and "it'll be VERY hard," etc. I don't want to stop him from pursuing his career but I now cant help but worry we'll grow apart. He assures me we'll be ok but I can't help but feel that isn't enough reassurance.