Thanks guys....

The concrete was to be used to strength with the fibreglass over it.
However, with the second option, I could just put the fibreglass. Not
sure about the weight though.. I suspect I might still need something
under it for strength.

As for the waterfall absorbing the water, I have been thinking about
this. Another issue is, ultimately, I want a little stream to a second
pond (where the pump will actually be). This will obviously mean a lot
more water is needed before it starts recycling. Waterfall needs
filling, the stream needs filling etc. The bottom pond will be deep
(for volume) but have shallow / rocky sides so you won't see the water
level drop drastically.

To aggravate the situation, I don't intend running the pump all the
time. This means I cannot "get it started and top it up".

Maybe I am being to ambitious