This is just a simple suggestion. Something easy to do. The small effort equates the small benefit. Please don't let this start a big debate or rant.
As most of us who have gone on guided fishing trips have experienced, water is an important issue. Unfortunately, the simplest way for most locales to provide safe, portable water is to provide water in 8 oz single use plastic bottles. But many places also have a source of safe drinking water at the lodge or on the live aboard. So I and most of the usual crew I fish with bring our own refillable water bottles. I bring my old Nalgene bottles and fill them each night before dinner. If there is a fridge, I ask to put them in the fridge to chill. I use them for water at dinner to brush my teeth that evening and in the morning and at breakfast. Then I top them off and bring them on the skiff. I estimate I probably avoid using 8 - 12 plastic bottles a day (that's in tropical locations like the Amazon or Bahamas). Over the course of a week, that's around 50 - 80 plastic bottles just for me. If my group consists of 6 people, that's approximately 300 - 500 bottles a week. Those bottles aren't tossed overboard but in most remote locations they are burned or simply crushed and landfilled. Not ideal no matter how you look at it.
We also encourage the lodge or skiff owners to bring refillable gallon jugs rather than the individual serving plastic bottles. We don't make a big deal out of it, we just point out the above estimate and how easy it is to reduce the number of plastic bottles. When the conversation is right, we also bring up the idea that the lodge owner might consider having refillable water containers (Nalgene or any of the hundreds of others) made up with the lodge logo and include them in the cost of the trip. It is then recommended that each person fill their water bottle and bring it each day on the skiff, etc. The client then takes the bottle home with them as a souvenir. I don't know how much a pallet of plastic water bottles costs versus the cost of Nalgene bottles for a season. But if the economics don't work, I think most fly fishers would be more than happy to bring a refillable water bottle with them if the lodge or skiff owner requested it. I'm sure there are some who expect the entire trip to be catered and their every demand provided for...but they would be a small irritating minority.