I've tried bottle cages under the saddle and they bounce out so went back to handlebar mounted. At least I can see them making a bid for freedom. I've recently angled the cage so that the bottom of the bottle is nearer me than the spout (if that makes sense) it seems to have worked but I've also gone to 500ml bottles. They seem to stay put.
I used to take out 2 large bottles and would struggle to drink it all. Yesterday I went out for a long ride (85 miles) I took two 750ml bottles out. It was hot, I rode over shap and kirkstone pass. I stopped twice once for a sandwich and i drank a 500ml bottle of bought water, I had a swift pint about ten miles from home. I actual brought back half a bottle of water. Neither of them had been refilled.
I'm not dead, i didn't have a raging thirst. If I needed it I'd drink it.