Hi! Do you mean a SMD LED? Can you share some pictures with us?
Anyway, when it does have water damage, the best thing to do is to check for corrosion and clean it roughly with isopropyl alcohol! Make sure that all the corrosion is gone and everything looks clean again!
Hopefully after this it works fine. If a component need to be replaced, then it's for the best to replace it before you are going to try it again.
Not sure if there are any parts available for the Pokemon Go Plus.. When it's a LED you can probably find a replacement part as those are universal (you do need the right LED off course).