Keep in mind that the biggest reason for deforestation and water pollution is actually because we have too many people on this planet. If you want those things well-preserved, we shouldn't go above about 3 billion people across the planet. The fact that meat is an inefficient process when you look at a single dynamic (either pounds of edible food or calories of edible food) is actually a secondary consideration.
Importantly, facts that pro-vegetarians refuse to acknowledge include understanding that food is complicated. To be healthy under a vegetarian diet requires that you have a good variety of vegetarian foods in your diet and artificially produced vitamins, minerals, and amino acids (some of the micro-nutrients vegetarians claim are in vegetarian food are actually analogues which are not the same thing). To be healthy with meat in your diet is pretty dirt simple (you can actually get everything your body needs with a pure meat diet if you include more of the animals such as liver which will cover your vitamin C needs if you don't massively overcook it), and doesn't require a great deal of variety.
Furthermore, back to the population issue, going vegetarian is only delaying the inevitable. We already farm enough of the planet that we are having major ecological impacts to the environment and, assuming the population doubles (which is likely unless we address overall wealth inequality across the world quickly...people in better economic conditions with reasonable quality of life always have fewer children, population would stabilize), we will continue to drastically have a negative impact to the environment.
Finally, it is important to note that part of the inefficiency problem is that cattle raising is typically done on bad land. E.g. cattle raised in East Africa are on bad lands with bad grasses and have, literally, a hundred times worse ecological impact than cattle raised on feedlots in the US. Planting better grasses would improve the situation drastically.
http://www.npr.org/sections/thesalt/...better-pasture