I like to make infusions with water in the mix that I separate out later, I was wondering, can I use that water, after draining and straining, to make marijuana tea?
Well, technically you can, but you probably won't want to. For one thing it won't taste very good. It probably won't get you high either as the cannabinoid containing trichomes bind to the fat that you infused, presumably butter or oil.
The bottom line is, when we add water to the mix when making infusions, we do so to leach out some of the green flavor and chlorophyll. While I applaud your thrifty use-it-all spirit, in my opinion, the best thing to do with the infusion water after draining and straining is to use it to water your plants. Otherwise toss it.
A better option for making marijuana tea is to use decarbed leaves, trimming, or even stalks and stems for mild tea. And you don't need to infuse for hours, just make like you would any other brewed tea. For best results consume with milk or cream or drink accompanied by a food that contains fat, as cannabinoids metabolize better in the presence of fat.