If your opponent activates "Fissure" while you have control a monster with a lower ATK "Shark Cruiser" on the field, you can activate "Book of Moon" on that monster, so it destroys "Shark Cruiser" instead, and it will activate its effect.
Similarly, if your opponent activates "Widespread Ruin", "Smashing Ground", etc., you can do the same trick so that "Shark Cruiser" is destroyed and activate its effect.
This card is great in "A Legendary Ocean" Decks to counter "Gladiator Beasts". "A Legendary Ocean" allows it to be set for no Tribute. Its defense, even without the boost from "A Legendary Ocean", keeps it from being destroyed by battle with any "Gladiator Beast" that can be Normal Summoned. While "A Legendary Ocean" is on the field, "Shark Cruiser" is even safe from being destroyed by battle from "Gladiator Beast Gyzarus", as well as the entire "Monarch" series.
Use "A Legendary Ocean" and "Water Hazard" to special summon this card in face-up defense, decreasing its chances of being destroyed by battle or destroyed while face-down.
You can use this card to Special Summon 2 "Spined Gillmans", giving you two 2100 attackers.
You can use this card to Special Summon 2 "Dupe Frogs" to prevent your opponent from attacking.