This achievement was also a pain the rear. To get it in theory you have to end the season with all five of your crewmen having max morale (a completely filled yellow meter). I did this on several occasions and no achievement. For other people they had no problem getting it. What I did was make a save right at the end of a season, kept ending the season and if no achievement I would reload my save file. After about 6 or 7 tries of this maybe more the achievement finally unlocked.
Morale Tips: First start by filling your roster with low% fishermen such as Burt or Roxanne. The reason behind this is that the greenhorn's morale will fill up much faster than the more experienced people. Kenny for example takes forever to fill up. First and foremost to raising morale is catch lots of crab. If you are hauling in tons of crab their meter will fill pretty quick. Make sure to give your crew breaks if they ask for them and don't work them too hard for too long. If you notice their red meters getting low, put the little guys to sleep for a while. Play on easy mode because this lessens the rate at which morale drops. You can also add things to your boat that will help such as a tv. Make sure you have someone who can cook onboard because they will be pretty pissed if they don't get some chow.