The OED has the following general meaning of seaboard:
The line where land and sea meet, the coastline; the sea-shore or the land near the sea, esp. considered with reference to its extent or configuration.
The first citation of this seaboard is from 1788:
"The Gnats are almost as troublesome here, as the moschetoes in the low-lands of the sea-board."
The OED doesn't seem to draw this connection, but I imagine this meaning came about as an extension of another meaning for seaboard, which is:
With prepositions a, at, on, to seaboard, on or to the seaward side (of a ship, etc.). Obs.
If you connect that meaning with other sea terms like overboard, it would make sense that the seaward side of a ship would be called a seaboard. And then saying that the coastline is essentially the seaboard of a landmass is a small jump.
Nowadays, it seems like "Eastern Seaboard" has become an idiom or set name for a certain region, and we don't really use it in the general sense at all (although I don't know anyone in the fishing industry).
I think it's mainly a BrE expression--a variant is all over him like a rash--commonly used to denote the excessive attention one person gives to another, usually when they've only just met, as a result of a sudden sexual attraction. I suppose it could be used about either sex, but I've only heard it used about a female in pursuit of a male. And the suit metaphor makes it tend that way in any case. Why it should be a "cheap suit" is less obvious, but one characteristic of an expensive suit is that you don't notice you're wearing it. If it's cheap, it's likely to be ill-fitting, made of abrasive material, and continually making its presence felt.
Best Answer
"Stone cold" is a common informal adjective. Using it to modify "loser" is not remarkable.
(Note that references tend to say it means "completely cold", without giving the inference. In practice it can mean either "cruel" or simply "absolute".)