I been using this under seat bag and has been great for years. Even with my tool kit, phone, key etc in it. I drive down the interstate at 75 with it on the bike and it's still there.
https://www.amazon.com/gp/product/B00A3W8FFM
The rear support strap goes theough the seat rails and around and under the bag and supports the weight unlike some bags that just have it sewn on the top. I have the large.

For lose water bottles, I wrap a piece of velco around the cage/bottle. It still comes in and out when I want it too but makes it tighter.