Thing Type 421, Bubble Shield Monitor (also known as the Water Shield Monitor) is a monitor that gives the player the Bubble Shield upon destruction. This shield protects the player from drowning in water, damage from slime, or suffocation in space. When a player with this shield presses the Spin button in mid-jump, they will dash to the ground with a slight momentum reduction and then bounce off at a height higher than their standard jump. The bounce can be done again after bouncing off ground, making this shield and the Attraction Shield the only two whose abilities can be chained. If the bounce is done on a slope, the player will be sent in the direction the slope is facing. Azure Temple Zone is the only level in the game to feature this shield.
Checking the Ambush flag turns this monitor into a Weak Random Monitor, while checking the Special flag turns this into a Strong Random Monitor. Neither of these are recommended however as both WRMs and SRMs can't spawn Water Shield Monitors.
This Monitor type also comes in a gold variation, Thing Type 451, Bubble Shield Monitor (Respawn), (Also known as the Water Shield Monitor (Respawn)) which will give the player the Bubble Shield once popped, before respawning after about 3 seconds. SRB2 itself does not use the Bubble Shield Monitor (Respawn), but it is still available for custom maps.