There are several people around here in Central Florida who have converted pools to ponds. None of them completely drained the pool. I do agree with Craig's suggestion to lower the pond water and paint the coping tile and top foot or so with black pool paint. You'll not be able get plants to grow over the sides unless they are in pots. If you cycle the pond without adding any chemicals, i.e. let it run and run and run without added chemicals, most will start to evaporate out. Plus you are coming up on the CA rainy season, and that should help you some.
All that said, the best definitive answer comes from a water test. Your local extension agent can help you get one through the state, or you can find a for-profit testing lab online. Chances are being California and all, you won't find anything bad, but a test will tell you for sure, take the guessing game out of the equation.
What little I know about swimming pool conversions is that though they are big, they really don't make all that good of a koi pond. You can't get good filtration. If you are dead set on having koi, then you'll need to do a lot more research and look into putting a bunch of filtering devices in, it looks like the ground slopes off behind the spa? That spot would work.
The pool conversions around here are garden ponds with goldfish, gambusia, rosy reds, sailfin mollies, killifish and the like for mosquito control. Most of these ponds are not clear.
If you want clearish water, you might look into building or buying a rock trickle tower to put nearish the center of the pool.