I liked the torpedo by Bcheirif and printed one, which worked well, but it is hard and uses a lot of material for infill. I am cheap and it hurts to get hit with it. I have kids, so someone is always going to get beaned by it. I made a hollow screw together version of the torpedo so that you can fill it with water to maintain neutral buoyancy but use less material. And then I printed the top of the torpedo in TPU so that it was soft and didn't hurt as much to get hit with. You can print the top in whatever material you are using for the body as well it just might hurt if you get hit by it. I still don't suggest throwing the TPU version at someone, at least out of the water. In the water it glides nicely and doesn't really hurt. I added an M5 threaded hole to the top of the cap so that you can screw in an M5 bolt and add nuts to play with the buoyancy.

For printing I use a raft for the bottom of the torpedo as there is not a lot of contact area with the bed. The small base lets it glide better in the water, fewer trailing vortices. For the top I do not use a raft and neither part needs supports.