Work may be done in positive or negative direction. When a work is done in positive direction, it is called useful work, when work is done in negative direction, it is known as useless work. Any energy associated with friction produces useless work. That is, the work is done in negative direction.
Therefore, thermal energy produced by friction can be used to do work but in negative direction.
The easiest way to get a fairly accurate measure of your water flow rate is to time yourself filling up a bucket. So for example if you fill up a 10 litre bucket in 1.5 minutes, then your flow rate will be: 10/1.5 = 6.66 Litres per minute.
