The Blue Zone Grenade is a throwable item can be used effectively if you want to deal damage in a broader area for a certain period of time. Especially in situations where you want to lure enemies out of a building. The Blue Zone Grenade creates a round blue zone with a maximum radius of ten meters, lasting for five seconds. It maintains its full size for five seconds, then decreases and disappears over another 5 seconds.
~ In-game description
Update #56 (Season 13 - Update 13.2)
The blue zone gets as big as ten meters in radius and lasts for five seconds.
It remains in its full size for five seconds and then shrinks and disappears for another five seconds.
It does not explode underwater.
But, when activated near water, it gives damage to players inside the water.
If you are already in the Blue Zone and are in range of an additional Blue Zone Grenade, additional damage will be applied.
Blue Zone + Blue Zone Grenade = Both damages apply
Blue Zone Grenade + Blue Zone Grenade = Both damages apply