Some bird species routinely use a tactic called kleptoparasitism to obtain food. They would just as soon steal food as find it themselves. Gulls may be at the top of the list for using this tactic, although eagles are also known for this. I have seen gulls expend much more energy chasing another gull with a fish all over creation rather than go catch a fish themselves.
One tactic they use in the winter is to fly over a raft of scaups or buffleheads and wait until one of these diving birds comes up with a crustacean or shellfish and try to steal it from them. The ducks know they are constantly trying to do this so, when a gull swoops in, they will often dive underwater. That is what is going on in this photo. The splash under the gull isn't from the gull, but from the duck that is diving back underwater. You can see the tips of it's wings as it is going under. If you look closely, you can also see the scaup to the right of the gull has something from the bottom in it's mouth also.
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