The showers are down near the dive pool, after the members only showers. (Picture below)
I used an app called Class Pass for City Gym. It has a trial, so I was able to go for free. I assume they would do a casual entry as they have separate casual and member lockers. But the don't mention the price online.
Tried the olympic park showers today! I like them! Went for a jog in local parklands , then had a shower before and after a swim in the competition pool. Someone was having a peak at me from change room first time, second time I had a lengthy shower naked with one other. Good water pressure and a turning tap instead of button which is always good. Urinal just a couple of steps from showers. Also seems to be only adults in there as kids tend to use to playpool/ little lap pool at the other end of the complex.
Just so others can find it easier it's called "competitor changerooms" Like you said next to the dive pool, on the right hand side walking toward it. dive pool area was closed off but was able to access from the back corridor.
There's so many changerooms in that place it's insane. Only seems to be one good one though, the other changerooms are cubicles from what I've seen.