It looks like a bit of artistic licence was taken (gosh :-) ).  You are correct that, the bags would not float that high up -- at least if I'm correctly viewing them as having only a tiny air pocket at the top.  

The physics is simple: take the total mass of the bag+fish+bagwater, and calculate the equivalent volume of seawater for the same mass.  That's how much volume the bag must displace, so if the bag 'assembly' has low enough density, the remainder of the bag's volume will be above the seawater surface.  It's identical to the way icebergs float.  

As a pal of mine says in situations like this,  "because movie science magic" .

Could fish in plastic bags filled with water float like that?
On Linux press F9 return a correct 

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On unix(solaris or hpux) return

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How to set correct tilde on those systems?
