Ducky,
I am afraid there is little possibility of the roof holding swimming pools of any size safely.
Depending on the construction detail it may be possible to spread the load of a tub over sufficient area to make it safe, or even a small pool if you could position it over or near a strong support below and you did not require sufficient depth of water for an adult to actually swim.
However a plan and detail of the construction would allow a definitive answer.
Regards
Norman