An explosive device contains a water-reactive material that ignites upon exposure to water. The water-reactive material ignites a water-activated fuse that has a predetermined burn rate and length. The predetermined burn rate and length allows the device to sink to a desired depth before exploding. Hence, the device explodes after a desired period of time and/or at a desired depth. Defense against underwater swimmers is an advantageous feature of embodiments of the disclosure. The device can use a forty millimeter (40 mm) form factor, which permits launch of the device from convention grenade launchers.