I would not recommend replacing the logic board, as it simply does not make any economical sense.
It is unlikely that the logic board is damaged, it's just the keyboard wiring layer beneath is shorted by water. However you cannot replace just the keyboard (unless you rip it off destructively), you have to replace the entire top case assembly.
So why not try with a known good unit? I find it interesting that most people here prefer to try the repair anyway before even locating the exact damage.