The bug
Even though you cannot select blocks outside the worldborder you can still place a boat at that position.
MC-113420
Boats can be placed outside the worldborder
Resolved
Fixed
2
(Unassigned)
Confirmed
boat, item, world-border
Minecraft 1.11.2, Minecraft 17w06a, Minecraft 1.12.1, Minecraft 1.12.2 Pre-Release 1, Minecraft 1.12.2 Pre-Release 2, Minecraft 1.12.2
Minecraft 17w50a
Retrieved
Can no longer reproduce in 17w50a. Apparently this got fixed in some 1.13 snapshot.