They are not meant to be submerged for any length of time, and with a small amount of air inside they won't sink. I use the yellow bag for towels. It's large enough to hold a bottle of sunscreen and two rolled-up towels.
These are great, inexpensive bags.
All the bags are made of lightweight nylon that is coated on the inside with waterproofing. They have the fold over top with a plastic clip. The small blue bag is the perfect size for phone, wallet, and keys (have to keep that key fob dry!). I use the large green bag for other small items I want to get to quickly and easily. I keep the medium grey bag around to loan to friends who show up with their phone in a ziploc bag.
The clip on top of each bag makes it easy attach to a line or my clothing. In fact, the little blue bag has become so useful that I carry it around with me in daily life, like a lady would a purse. These bags don't hold air for very long which made me nervous that they might let water in, but so far everything inside has stayed dry.