Why does the shrimp stick to the shells

According to this study, frozen seafood has a density ranging from 972 to 1017 kg/m³. Thawed, raw seafood at 20°C has a density ranging from 1042 to 1093 kg/m³. Since the density of water at room temperature is about 998 kg/m³, it is certainly possible—and most likely normal—for one batch of frozen seafood to float while another sinks. Thawed seafood that has been brought to room temperature, on the other hand, should almost always sink since its minimum density (1042 kg/m³) is higher than water's. Therefore, I would only worry if the shrimp continue to float after they have been thawed. If they are shell-on, there is a chance that there could be air stuck between the shell and the body, though, which would of course be perfectly fine yet affect the density/floating.

Edit: What are the ingredients listed on the bags? Very often nowadays salt and salt-based preservatives are added to frozen shrimp in order to retard water loss. It could be the case that one bag has preservatives while the other does not. This could of course affect buoyancy by changing the density of the shrimp. (In general, one should try and only buy individually quick-frozen shrimp where "shrimp" is the only ingredient.)

Shrimp are extremely lean and have very little connective tissue, which means that the longer they cook the harder they get. Fresh shrimp, for example, only need to be fried for about one minute per side (depending on their size). This makes cooking frozen shrimp very difficult: By the time the outside has thawed and is perfectly cooked, the inside will still likely be frozen. If you cook them long enough for the inside to thaw and cook (e.g., 30 minutes), the outside will likely be extremely hard.

I would recommend thawing the shrimp before cooking, for example, by leaving them in the refrigerator overnight. Another quick way to thaw them is to place them in a bowl and run a stream of cold water over them in your sink. Next, thoroughly dry the shrimp, which will help prevent splatter and promote the Maillard reaction. Only cook the shrimp until they turn slightly pink and begin to curl on one side, then flip. Depending on the size, this should be no more than, say, two minutes per side. Then proceed with the remainder of the recipe. Also take into account if the recipe calls for simmering the shrimp in the cream sauce; if so, then this will continue to cook the shrimp and you will not need to fry them as long.