Exercise 3 solution

Water absorbs red light fastest, then orange and yellow; blue and green travel deepest.

At 1 m: the object still looks red, since almost all the light, including red, is still present.
At 20 m: the red has largely been absorbed, so the object looks dark brownish-grey or black; there is little red light left to reflect.
At 100 m: almost only dim blue light remains, so the object looks dark grey or black (a bright torch restores the red, because it supplies red light).

The depths are approximate and depend on the clarity of the water.