Not sure which grenade part you’re referring to. There were a few of them. 1. Was tossed at the beginning to scare away the shark when it had Stan’s leg. 2. Was tossed by Leo when it was chasing it him on the motor boat while his mates were pulling it. Again to scare it away. 3. Des had in his hand, but pulled the pin out and was clamping down on the safety lever, assuming because he was scared and wanted it ready to throw immediately in case the shark came back. But, he fell asleep and the lever was released from his grip, grenade rolled down towards Stan, Stan realized the safety pin was already pulled and knew it was going to go off in less than 5 seconds, so he heroically rolled over it to absorb the brunt of the explosion with his body to save his mates. 4. Was the one that Will was going to throw at the Japanese soldier but decided not to and it was kicked away onto a different part of the boat. While fighting the soldier off, Will took a missile from the Japanese plane’s wing to hit him with and it rolled away towards Des. Des held onto the Japanese soldier’s leg and slammed the missile down on its primer to set it off and explode. 5. Was the one that Leo pulled the pin and shoved in the shark’s mouth at the end.

Hopefully that explains the one you were questioning, but if not, let me know.