Beat together the margarine and vegan sugars. Electric mixers are unnecessary. Add the well-mashed banana and mix well, then add the vanilla, then the water. The water will try to separate; keep mixing with a figure-eight motion and add the dry ingredients, in the order above, bit by bit (say, by half-cup increments).
The final batter should be almost too dry to hold the chocolate chips; if it isn't, I've misrepresented the amount of flour or oats you need and you should add more or less, according to the problem. Dry batter is good, since any chips that fall off the batter can go directly into your mouth, and no one will miss them.
Bake them for 9-10 minutes in a pre-heated, 375-degree F oven on an ungreased cookie sheet. Cook them longer if you prefer a hard cookie, but be aware that bananas only approximate the action of an egg in the mix, and you can get cookies as hard as the cookie sheet if you aren't careful.
Let them congeal on the sheet for a moment after removing them from the oven, then cool on a plate or a wire rack. Store in an air-tight container. They taste even better after they sit overnight or in the fridge.