In ZEUS: Spawn a group of divers on bluefor, spawn a group of divers on redfor (specically the un-mounted groups from the drop down menu), have them meet up for combat, randomly you will notice explosions in the water, under the water, above the water, everyones armed with underwater rifles. The grenades/explosions will be delayed for 20 seconds or so, the engagement must last longer then 20 seconds.
In EDITOR: Same as above, almost identical effects.