I use a Camelbak and it gets heavy on my back. When it is filled (about 2.5 liters or close to 6 pounds), it is uncomfortable on my back. But, I drink much more water with it and I put all my valuables in it so I always have them with me. I rarely run low on water.
I can't imagine not having some kind of pad to sleep on. I use a thin Thermarest and was thinking of getting something even thicker.
Heavy cameras are a personal choice. If you really value the photos it takes, carry it. If you are only going to post the photos on your website, get a lighter camera or use a smart phone.
You clothes will get wrinkled and dirty no matter how careful you are. I use gallon ziplock bags to hold my folded clothes in my panniers. It helps but not much.
The amount of water I carry depends on the day I have planned. I always ride with my camelbak and have a pretty good idea of how long I can go with a full pack. I prefer not to carry extra water. If possible, I plan to get water along the way. Many times, I have stopped at a house and asked for water which has never been refused. On a recent trip, I got water from passing motorists, both requested and not. This is something you have to work out for yourself.
I don't lock my bike bags and I rarely let my bike out of my sight. A quick way to do it is with zipties (need to be cut off). There are lockable metal meshes you can get.
U-locks are heavy and unnecessary, in my opinion. Here is an article I wrote about locking you bike on tour
I either have electronics that use replaceable batteries (camera) or use available electrical outlets at bakeries, coffee places, restaurants, motels, or anywhere else to charge my iPhone and iPad.
Some articles you might find interesting:Taking less weight on tour
.Bike touring trade-offs