I’m guessing the reason the item fell is because its collision region is not in contact with the ground, so its falling until the collision region hits the ground. Since you actually want it floating in the water, it’s probably not a big deal.
It’s best if you can set the origin of your object in your modelling program. I just played around a bit with MagicaVoxel and it looks like it has no way to do that, which is a problem! I also tried exporting from MagicaVoxel to a qb file and opening it up in Qubicle. It looks like there are some artifacts that shouldn’t be there, the entire object is rotated and the model origin is off from where you’d expect it to be.
Maybe have a look at StoneVox? It’s still possible to get Qubicle 1.6 Stonehearth Edition (one of the questions at bottom of page has a link) too.