If a card is used to remove or destroy a face-up "Umi" on your side of the field, if you chain "Forgotten Temple of the Deep", this card remains active on the field.
This card saves your life points when you have to play a monster with little ATK in attack mode for its effect to activate (i.e. "Deep Sea Diva").
If your opponent has this card out, note that it only prevents damage from your attacking monsters. If you use ATK/DEF altering cards during the damage step when your opponent attacks, such as "Skull Dice" or "Honest", you can still inflict battle damage to your opponent.