It was called boat because they didn't handle very well, and were tricky in tight situations. It was derogatory, though it was said in that tone of voice for sarcastic reasons.

Among young people there does seem to be such pressure - I hear plenty of folks around my age (late teens, early 20s) talking about a car being a boat as a downside.

Not growing up in the US and only seeing cars like that on TV, I always assumed "boat" was referring to the handling more than the size.