It should be a trap door spider.
When I first found one of them in the pool, I thought it surely must be a funnel web. The Sydney funnel web spider is one of the deadliest spiders. Luckily, we don't get that in Melbourne, and the Victorian funnel web shouldn't live in our area either. So, I sent a picture in to Museums Victoria. They offer an identification service, and told me that these are trap door spiders. Not considered dangerous. Their bite is still supposed to be quite painful, but they are not aggressive.
Certainly another motivation to keep the pool as clear as possible and to remove leaves before swimming. They can survive quite a long time under water, up to now they were all still alive when I fished them out...