RossT

Those are definitely water boatmen. I've seen them in pools and lakes before. They're kind of fun to watch. We never had so many that is wasn't easy to get rid of. Usually we could get them all with a skimmer net.

You may be able to siphon it out with a garden hose although that sounds like a lot of sand. I've had above ground pools my whole life and what we do is create a whirlpool current by walking around the pool. This should push most of the sand to center which will make it easy to scoop up and out. The remainder could be siphoned or you could get a sump pump for a better suction to suck the sand out.

I think whatever you do, don't run the filter all winter long while it's closed. You'll not only save money on electricity but probably extend the life of your pump. You should talk with the local pool supply company about whether you need to add anything to the water to make it easier and quicker to balance and open in the spring.

Usually with that much rust and that big of rust, you'd find some metal that was causing it. What is the average size of the stains? It's hard to tell from the pictures. If it was a leaf, you should have seen the leaf on the bottom or in the filter. Do you have any leaves that are falling in your pool?

What about those solar rings that I've seen at some pool stores, would those work? I know that heat could still escape between the spaces but it may be good for an odd shaped pool. Plus they are probably easier to put on and take off the pool and store as opposed to a large solar blanket.