As long as the bricks are clean of scum and algae, there should be no
problem with using a 4 or 5 to one mix of sad and cement. Break off
all the loose and friable material.

Just give the surface to be rendered a quick wire brushing and then
slap it on.

Being wet, the bricks will allow better adhesion than if they were
dry. Perhaps someone will be along to recommend and additive for
waterproofing but I'd steer clear of such chemicals in a fish pond.

I wouldn't even use Feb and certainly not use washing up liquid as a
plasticiser.

Hi,
Thanks for the reply - no its mainly decorative, few plants not always water
and definitely no fish, but I need to plan
for waterproofness in case one day he wants to fill it.

Thanks,
Nick