Instructions
Wait until the Energy bar of the Sim you want to drown is half-way. This will make it easier for your Sim to drown faster because of exhaustion.
Enter "Build Mode" and create a pool with a ladder. Enter "Live Mode" and have the Sim swim in the pool.
Enter "Build Mode" once again and use the "Hand Tool" to grab the ladder, then press "Delete" on your keyboard to delete the ladder. Do this for every ladder in the pool.
Wait for your Sim to become exhausted by swimming -- it will have a thought bubble with a pool ladder above its head. Once it is the Sim's time to go, the Grim Reaper will come and the Sim will become a ghost.