This is why water beds are amazing
Winter you put memory foam to keep you from the cold water, in summer you sleep on the cold water.
Only downsides to a water bed are: 1. Heavy 2. You have to add chemicals to your mattress as regular maintenance 3. It can't really be extra firm (but a lot more firm than people think)
The water will steal every bit of heat from your body, but you'll stay warm with a blanket