Water Bomb (object)
From the Super Mario Wiki, the Mario encyclopedia
Water Bombs are large, cannonball-like objects in Super Mario 64 and Super Mario 64 DS. They are shot out of cannons on the Bob-omb Battlefield course, specifically the unusable cannon on the mountain (along with the one at the start in the remake), and in the Battle Fort stage in the DS remake where they fall out of the sky at random. They typically land in the larger field areas of Bob-omb Battlefield and act like large, bouncing water balloons, exploding and dealing damage if they come into contact with the player character. After two hops, the Water Bomb will dissipate the next time it hits the ground.
Though the player can see the Water Bombs shoot out of cannons, they will not fall to the ground and keep moving up into the sky. Instead, Water Bombs will appear from the sky and fall on or near the player.