What they were trying to do did, indeed, seem a little crazy. There were two great abominations fighting on one side of the Black Turtle, and Nephis wanted to dive into the water on the other side to steal some of the turtle's meat.
Because she was hungry.
But, of course, it was really not that unhinged of a plan. She had not suggested it out of a burning desire to satiate her belly — rather, it was out of necessity. The longer they were to go hungry, the weaker they would become. The weaker they became, the slimmer their chances of survival would be.
Neither Nephis nor Sunny knew if they would get a better chance to harvest the Black Turtle's meat than now, when the Azure Serpent was distracted. In fact, they did not even know if they would live long enough to enjoy the fruits of this perilous decision.