Don't know what else has to fit on top of your rear rack, but the sleeping pad and tent could both go on there with the long dimension lined up with the bike so they'd be in your wind shadow.

For water I usually just carry two or three bottles in holders on the frame and also have a lightweight 'Water Bag' that's a double-layer plastic bag inside nylon fabric. The Water Bag is normally carried empty and folded inside a pannier, but can hold over a gallon of water when needed. Handy for use in campgrounds where the water spigot is some distance from my tent site or for crossing areas without many water sources.