Why are there no seas in U.S.? There are Gulfs and Bays, but no Seas
(except Caribbean to a degree). Other countries have plenty of seas
(e.g., Adriatic, Balearic, Tyrrhenian, Aegean, Okhotsk,  Laptev,
Karskoye, Barents, Arabian, etc seas). What guides that qualification?
Purely historical factors? What are they? My husband suggests that
only those places that supported substantial native sea-going named
bodies of water around them seas, whereas others do not. That does not
work smoothly for everything (e.g., explains lack of seas in Africa,
but not (strictly speaking) presence of seas in Australia and
Antarctica).

In California, we have the Salton Sea, which is the state's largest lake.
http://www.saltonsea.ca.gov/about/about.htm

Also, there's the Bering Sea off the coast of Alaska, which is of
course one of the United States.

Bering Sea does not qualify, as it was named by Russians prior to selling Alaska.

Salton Sea I suppose sort of qualifies, although, say Caspian Sea  -
many consider the name incorrect (they consider it a lake). Still,
"Salton Sea is an inland saline lake"...

"The early Appalachians formed about 400 million years ago when what
is now North America collided with Africa and Europe near the equator.
Prior to this, only northernmost North America was dry, and what is
now Florida was off on its own, located deep in the southern
hemisphere. By 300 million years ago, inland seas had drained, the
climate was warm and moist, and the Southeast was covered by lush
forests. Over time, plant and animal remains transformed into the coal
deposits that underlie much of the region today.

Between 225 and 65 million years ago -- the dinosaur age -- shallow
seas covered much of North America's interior as well as parts of the
Southeast's Coastal Plain. Few dinosaur fossils have been found in
this region. Much more common from this time are marine animal
fossils. Sharks, crocodiles, and long-necked reptiles called mosasaurs
left their remains, as did ancient snails, clams, and sponges. The
flood-prone lowlands were underwater again following the dinosaur
extinction. Some of the more notable fossils, from about 40 million
years ago, are bones from prehistoric whales that grew up to 70 feet
long.

During the Ice Age, which began 2 million years ago, sea levels
dropped. Florida, which had been submerged for 100 million years or
so, was dry once again. While glaciers didn't reach this region, lots
of Ice Age sediment, including bones and teeth of mastodons and
horses, was deposited in the Southeast by rivers like the
Mississippi."

http://www.pbs.org/wgbh/evolution/local/southeast.html

Don?t fret -- couple of hundred years time you?ll have a whopper right
in the middle of the States. Might I suggest it be named ?The
Profligate With Mineral Energy Sources Sea.? ;-)