If the kite is pulling a board its a sail and the board is a vessel. If the
kite stands alone it is an un-mechanically powered seaplane unless towed by
another boat in which case the tow boat is the responsible vessel. The
question of sail or no sail is moot. If it's 'capable or being used as a
means of transportation' it comes undere ColRegs International and National
in the US and Canada. Can't speak for the Inland Rules of Britain I
haven't read thier version since 1997.

Whichever, Rule 2 still rules for your own vessel. Regardless.

MST