Does.it stay on in the water

It is the only one, out several I tried, that actually stayed on for five days while swimming , showers, and hot tubs.

Is this 100% latex free?

Yeah, most likely. It doesn't say anything about latex on the box.

Can you use this on a popped blister?

It's used primarily for prevention or protection of rubbing.. so yes. But it won't necessarily heal your blister. You will want to apply the first aid that works for you.

How do you open this box????

Try this: with the label facing you, put your thumb on one side of the plastic container and your middle finger on the other side. When you squeeze the sides together you will create enough space between the top and bottom of the container to pop it open. Hope this helps.