I think if you are using a harness it would not be a bad idea for everyone to have one so she does not feel like she is being punished. I love the swimming pool idea, and is not just for kids! Don't forget a rubber duck :-) Sounds like a good idea on a really hot day, and you can use it as an excuse/use to cycle the water out of the tanks, rather than just pumping it over board.
We had all her pool toys floating around the cockpit at various times, along with a bucket. It was nice for mom and dad too...cool feet on a hot day.
To be honest, we didn't use the harness much, preferring to use a life jacket when the Bay started to get "nautical". It didn't take much to keep her in the cockpit. We explained that the lifejacket and harness were because she was small and not strong enough to swim while we maneuvered to get back to her. Even at 3, she understood and didn't need us to wear ours. Anyway, mom was never far away.
Whenever conditions got too vigorous, she went below on her own, so it was never an issue for us.
Our rule for wearing lifejackets was that when she could tread water for 10 minutes, she didn't have to wear one. That was in the days before affordable inflatables. Now, we wear ours at night, anytime we reef, or when there are whitecaps. We didn't use a net; we didn't think it necessary.
Dealing with kids on a boat has no set answer. You just have to find what you are comfortable with and what will work with your child. Everyone is different and what works for some won't work for others. And kids change, so what works today, may not tomorrow and vice versa.