HamNCheese writes:

Do I need to stop filling the pool below the plaster hole to prevent
water from getting inside, and behind, the plaster?

No, don't worry about that.

Or is this a
repair I can deal with another time and I can fill the pool up to the
top for now and then just drain the pool below the hole when I'm ready
to fix it?

Yes.

Assuming there's no crack in the shell that's going to leak water, in
which case you could just do a quick and sloppy patch to stop that.

How hard is it to patch the plaster, is this something I can do myself
or do I need to have a pro do it?

Patching with "cold joints" has from ancient times been nearly
impossible to screed and finish properly, but if you're willing to pay
for diamond polishing tools, you can do a perfect job of it by putting
on too much plaster and polishing off the excess. See my description
at:

http://www.truetex.com/pool.htm