This is nonsense unless you can offer a geological, not lexical, reason why the relationship between the Gulf of Mexico and the Atlantic Ocean is meaningfully different from the relationship between Long Island Sound and the Atlantic Ocean.
Geologically, the Gulf of Mexico is on the Norfth American Plate, but the Caribbean Sea is on its own small plate, intruded between the North American and South American plates.
The logic to this peculiar statement which the OP asks about, so far as I’m able to grasp it, is that the “coast line” jumps across from Rhode Island to the tip of Long Island, treating Long Island Sound like a river estuary or Delaware or Chesapeake Bays.
I’ve never heard of defining the boundaries of bodies of water according to what geological plate is underneath. Does it have any effect on the actual physical properties of the body of water above?
That makes it sound like it’s merely an issue of units of measure. So, as asked above, do the Mediterranean countries not have coasts?
I wasn’t contradicting, what you said, I was extending it. Like you, I don’t see how Long Island Sound different from the Gulf except in size. Heck, I don’t see how it is any different from neighboring Block Island Sound or Buzzard’s Bay, Vineyard Sound, and Nantucket Sound further east.
You’d asked about geology, so I provided what useful data I could. AFAIK the only real influence of plates is in questions of depth and seabottom topography (trenches and rises, or level plains?) and their influence on water surface conditions.
Italy, Greece, Croatia, Libya, etc., do not have Atlantic Ocean coastline, the Mediterranean being regarded as a distinct body of water (which in fact it is, being along with the Black Sea the last remnants of the Iapetus Ocean of Mesozoic times).
I completely agree that it’s pretty much arbitrary where one makes those leaps to say “this embayment counts as coastline, this one does not.” I simply tried to identify what rationale might be used to say that Connecticut has no ocean coast.
Rhode Island is the Ocean State. Massachusetts is the Bay State. Take a look at the maps and explain that one.
Darn. I liked having a coast.
I suppose I should have said oceanographic.
Ok, let’s do it with Canada:
Provinces and territories with Coastline
British Columbia (Pacific)
Yukon (Arctic)
Northwest Territories (Arctic)
Nunavut (Arctic)
Nova Scotia (Atlantic)
Newfoundland and Labrador (Atlantic)
Shoreline, but no coast:
Manitoba (Hudson Bay)
Ontario (Hudson Bay, several Great Lakes)
Quebec (Hudson Bay, Ungava Bay, Gulf of St. Lawrence)
New Brunswick (Bay of Fundy, Gulf of St. Lawrence)
Prince Edward Island (Gulf of St. Lawrence)
No shoreline at all:
Alberta
Saskatchewan
I don’t think it’s to do with Long Island Sound being classed as an estuary.
As per Wikipedia, I think the “technicality” is based on the fact that Connecticut is cut off from the open ocean by New York - see the map here.
New York state extends “offshore” from the CT coast so that there is a tripoint with Rhode Island. A small portion of RI territorial waters completes the “cutoff”. In other words, because NY has a border with RI, CT cannot have a border with the ocean (the four-colour map theorem…)
So basically, you can’t get from CT to the open Atlantic without passing through either NY or RI. I agree that this is a stupid distinction though, and I would say it has a sea coast.
In this context the issue is not geology, but geography. You need a geographical reason, not a lexical reason, and not a geological reason.
As I said in my previous post, the geographic reason is that New York and Rhode Island form a contiguous barrier cutting off Connecticut from the ocean.
You will be able to support this? I’ll give you “lame”, but am intrigued about “pedantic to a fault”. Cheers.
Hint: QI isn’t the entirety of UK knowledge-based game shows
I’ve long advocated for the annexation of RI by Connecticut. All the more reason now.
Hold on a sec – Annexing RI means we not only get Newport, Brown, and Federal Hill, we also get the Patriarcas and Ciancis. That doesn’t sound like such a deal to me.
But if Rhode Island is annexed, all those size comparisons (be it for icebergs, asteroids or Hollywood egos) of “this is X times bigger than Rhode Island” won’t have quite the same oomph.
What has QI got to do with this discussion?
If ever there was a case for “be careful what you wish for, you might get it,” that is it.
That’s true… but we could use a great big parking lot… 
Sorry!