You should read Death In The Long Grass! A violent book about a hunter in africa (nonfiction contains disturbing and violent material rated M for blood and gore and smoking)
The grooves are not what makes shrapnel they are for the user to hold on in mud and rain. The smooth concussion type are designed to cause more of a shockwave type explosion and will still produce shrapnel, however fragmentation grenades often have pre formed shrapnel inside the casing, also contrary to popular belief the handle on the side is for preventing detonation until you are ready to throw.(you pull the pin and hold the handle down while the handle is down the grenade is not a ticking time bomb when you let go of the handle you have 10 seconds on most grenades until it blows up)
Theoretically with -22% of your body weight in helium and a stride frequency of 1.920 HZ ±0.068 you could stay on top of the water with flippers