Don't go diving looking for those, you won't find much
Cardigan Bay [5]
Atlantic [3]
Baie de Douarnenez [1]
Baltic Sea [4]
Lake Svetloyar [6]
I think the answer is
I found this answer by
Theorizing, based on the title, that we might be looking for lost cities, specifically sunken cities, which are former cities that, according to myths, are now at the location of a currently-existing body of water.
Indeed,
Cantre'r Gwaelod is supposedly a lost city located in modern-day Cardigan Bay.
Atlantis is in the Atlantic.
Ys is in Baie de Douarnenez.
Vineta is in the Baltic Sea.
And Kitezh is in Lake Svetloyar.
And finally
Given the number n, we take the nth letter of the respective sunken city. This gives us Cant(R)e'r Gwaelod + At(L)antis + (Y)s + Vin(E)ta + Kitez(H) -> RLYEH -> R'lyeh.
The answer is apt since
R'lyeh itself is a fictional lost city in H.P. Lovecraft's short story, "The Call of Cthulhu", where it is located in the southern Pacific Ocean. (More details about the city in the link in the first spoiler.)