When Should You Speak Up?

So I was talking to an acquaintance of mine the other day who is very involved in holistic healing. We were talking about how the holistic approach to the body should mirror the way we treat the earth, so he agreed that being green is very important. But then, he proceeded to tell me that he had been invited to participate in a holistic health convention that takes place -- wait for it -- on a cruise ship!! I'm sorry..WHA??? I don't get it. How can anyone who purports to be holistic and green set foot on a cruise ship? Don't people know that cruise ships basically have destroyed our oceans? Here is a fact from Oceana.org:

Cruise ships generate an astonishing amount of pollution: up to 25,000 gallons of sewage from toilets and 143,000 gallons of sewage from sinks, galleys and showers each day.

That's just one astonishing fact. There are many more disturbing facts about the impact of cruise ships on our oceans. (For more: http://na.oceana.org/en/our-work/stop-ocean-pollution/cruise-ship-pollution/overview or http://www.odec.ca/projects/2006/kutp6p2/)

So, at any rate, being the OCD-ish greenist that I am, it was difficult for me not to lecture this acquaintance! But I didn't. I thought: What difference will it make? He's not going to cancel his appearance just because I launch into a diatribe about the evils of cruise ships.

But isn't this the dilemma we all face? What if just one person said just one thing to one person, and that one person made one different choice? If that happened just once a day, it could add up over time, right? Where do we draw that line between being a responsible inhabitant of this earth and a good conversationalist? I guess it all comes down to priorities. I'm not sure I had mine straight the other day. Maybe I'll send him an email with a link to Oceana.org... What would you do?