Van,

A possible candidate:

- 1363: Battle of Lake Poyang; Chinese used a similar technique to the one you described.

The main action of that day (31 August) would involve the creation and launching of fire ships by the Ming. Small rafts and fishing boats were loaded up with bales of straw and gunpowder, set aflame, and launched toward the enemy fleet. Dummies with armor and weapons were placed on the fireships as well, to aid in confusing and tricking the enemy. Due to a favorable wind, and the tight formation of the Han fleet, the fire ships were very successful, and many Han ships were either destroyed or suffered extensive damage.
There might be an earlier Chinese example because the Chinese introduced gunpowder to the battlefield in the 10th century. And though not 'explosive', there are of course many historical examples of setting fire to some kind of vehicle (wheels, wagons, ships, animals, etc) and employing them against the enemy.