I used to work for an environmental consulting firm that was hired by a cruise ship company to do third party monitoring; as a result, I have worked with many environmental officers.
What GB said is true. In addition to that, I believe some EO's are also responsible for health related issues on the vessels.
On top of everything, the hours are very long. They work around the clock and at least 12 hours a day. Their sleeping pattern is dependent on the docking time. On early morning port days, they either don't sleep or get up extremely early in the morning. There are no weekends or holidays, and they rarely get to enjoy port days. There was only one time that I can remember when the EO actually had plans to get off the ship, and it was to watch an evening movie (given that I typically see them at turn around ports).
Understanding multiple languages will probably help on the job, as not everyone encountered at each port speak English. If you happen to become an EO on an Alaskan ship, you will also find yourself with an ocean ranger on board, monitoring what you do. Some EO's have to do wet lab on board, but that depends on the company/ship and how the tasks are divided.
This post is by no mean to discourage the EO dream. Despite learning about all these, a part of me still wanted to try out the sea life; however, being in a relationship makes it even more difficult and thus a no go. On the bright side, the EO's I worked with have a flat screen TV and their own room!