We will swim a minimum of 11 pools of varying lengths.
I have used the doubled (one sealed and then placed inside another, sealed) trash compactor bags numerous times. I have always taken 2 more as backup, and have never used one of the spares.
Remember... the trash compactor bags are placed INSIDE your pack. So the only things that you need to watch would be any sharp edges on items like stoves, etc. A sock can be placed over items like these.
The bags mentioed as available @ Larada's sound good, as well. Especially as they are reinforced. As I do a lot of rafting & canoeing, and already have one, I'll be bringing a Boundary Bag. Basically it is a dry bag with shoulder & waist straps, like a backpack.
Yes, your pack will get wet. But in the end of May it'll likely feel good.
Otherwise, in order that your pack will not be too cumbersome, go as light as possible. Synthetics and fleece dry super-fast in the hot, dry, May air. Wool, as well, is good.
"Do not burn yourself out. Be as I am-a reluctant enthusiast...a part time crusader, a half-hearted fanatic. Save the other half of yourselves and your lives for pleasure and adventure. It is not enough to fight for the land; it is even more important to enjoy it. While you can. While it is still there. So get out there and hunt and fish and mess around with your friends, ramble out yonder and explore the forests, encounter the grizz, climb the mountains, bag the peaks. Run the rivers, breathe deep of that yet sweet and lucid air, sit quietly for a while and contemplate the precious stillness, that lovely, mysterious and awesome space. Enjoy yourselves, keep your brain in your head and your head firmly attached to the body, the body active and alive, and I promise you this much: I promise you this one sweet victories over our enemies, over those deskbound people with their hearts in a safe deposit box and their eyes hypnotized by desk calculators. I promise you this You will outlive the SOB's"