How do I cook Basmati rice without soaking?

Basmati doesn't need to be soaked, and anyway the water that gets absorbed in soaking would get absorbed in boiling instead. This is because the rice needs to do two things: hydrate, and heat, and the amount it needs to hydrate is fixed.

Rinsing is common and is much more wasteful of water than soaking. In fact if you measure the water, and soak then boil covered in the same water, all the water you use a ends up consumed (except a tiny amount lost to evaporation). 2:1 water:rice by volume should be about right for basmati, and cook gently.

Rinsing affects the surface starch, and it takes quite a lot of water to remove enough starch to prevent stickiness. You have to either tolerate some stickiness or find a good way to reuse the rinsing water, if you're going to save this. I wouldn't want to give too much starchy water to plants, though you could certainly give them some. It should be possible to use this water in cooking, to add to dishes that need a little thickening, but it shouldn't be stored for long.

Soaking rice speeds up the cooking by kick-starting the absorption of water before the rice even enters the pot. By letting rice soak for 30 minutes or so, you can reduce the cooking time of most rice varieties by about 20 percent. Soaking rice can also affect the flavor of the finished dish. Acetylpyrroline, the flavor component in aromatic rice varieties (such as jasmine, basmati, wild pecan, Wehani, and Texmati) that is mainly responsible for their characteristic popcorn-like aroma, dissipates during cooking. The longer your rice is over the heat, the less aromatic it will be. So by soaking the rice and shortening the cooking time, you get more flavorful results.