Crews are increasingly unhappy at sea, according to welfare charity The Mission to Seafarers.
It said a survey has highlighted growing problems with obesity, mental health and gender discrimination.
The charity's Seafarer Happiness Index shows satisfaction levels have dropped to 6.32 out of 10, a decrease of more than 5% year-on-year.
Many respondents reported high levels of boredom and stress.
Some criticised the shortage of facilities on board, while others complained of a lack of shore leave and access to reliable wifi.
Several female respondents reported gender-based discrimination and sexual harassment at sea, the Mission said.
Female crew made up 12% of those taking part - and they tended to be less happy than their male colleagues.
A few reported feeling lonely or isolated, especially when they were the only woman on board, the Mission said.
Seafarers claimed chandlers were sending "cheaper, lower-quality branded food supplies to ships, while others stated that while they wanted to exercise they found it very hard to either get the time or the motivation", it added.
The Mission’s director of advocacy and regional engagement Ben Bailey said: “In an industry that so often goes unseen by the vast majority of the global population, it’s so important that we do our part to show that these men and women are not forgotten and that we are always listening."
