There's no sea. Not one for a thousand miles and more, in any direction.
One wag told me that they were blown up here by residue of Hurricane Camille in 1969.
At the time, I almost believed that.
I recently saw the question posed on Ask!, and one respondent's answer astonished me: she claimed to be a lifelong resident of Denver and "I've never seen a seagull here in all my years. I think you have the wrong species in mind".
Apparently this lifelong Denver resident has never visited a lake, pond, river, golf course or grocery store parking lot in the Denver area.
Yes...Denver has seagulls. They look like seagulls. They fly like seagulls. They make noises like seagulls (kind of like a flying elk during rut, without the risk of a really gnarly elk-sized dump from above). And they scavenge like seagulls.
And like crows, magpies or other scavengers, they'll occasionally get brazen with their scavenging.
Worse...they're starting to teach other avian species ways to get back at all those spotless clean windows: