|We have stayed at the Natimuk Lake Caravan Park quite a few times over the years. In fact, we've experienced the full gamut of conditions and would say that it is only not so nice when the lake is completely dried up, and even then it is less dry and dismal than being at the crag. When the lake is full it is a great place to use as a base for climbing at the Mount. There is lots of bird life around and it's quite a lot cooler than camping at the crag, plus there is the bonus of being able to have a shower, and plenty of fresh water, powered sites if you want them - and if you don't, a neat little spot on the other side of the creek with a shelter that even has a fridge. The downside is that you are away from the crag (some see this as an upside) and have to drive in each day.
When the lake is full swimming is certainly a possibility but maybe not when it is down to the ¾ level, as I don't think the jetty would still be in the water. Depending on how long you are planning to stay, it can still be a good place to take a family, especially if they like poking around looking at birds. It's a pretty good venue to potter around on a bike too.