Protecting equipment submerged in water against earth fault - RCCB vs MCB
The appropriate device to protect against current leakage (i.e. appliances submerged in water exposing electric current) is RCCB. Essentially an RCCB detects any leakage currents, be it from a current carrying wire opening up under water or from a living being (person, animal or even fish) getting electrocuted due to whatever fault in the appliance.
On the other hand, an MCB protects against short circuit or current overload. And it protects the wiring.
In summary -
RCCB - protects the person (or living beings)
MCB - protects the wiring in the house
Special thanks to user253751 for commenting below with an example in which an RCCB would work but an MCB would not.