(1) The hidden underlying reference to squareness, and boat-like shape, of the heads on Singaporeans (originally designed by the Merry Boat Heads clan as a way to insult without having to face the penalties/risks of being "offensive").
(2) It also can be referenced to the greatest gaming clan in Singapore - the notorious Merry Boat Heads.
Being insulted as a boathead is probably the worst thing you can imagine - yet each member of the Merry Boat Heads is, ironically, able to be compared to that of a God; they are each that amazing.
(3) It's a professional troll word - used by only the masters.
You're a boathead!
Don't worry; The Merry Boat-Heads crushed you - it's expected since the start!