Just had a closer look at the waterplane that comes with that tub I used in MeshLab, and now I suspect LuxRender might have a problem with it. It looks like it consists of two back-to-back meshes sharing the same plane. I cannot imagine this to be compatible with LuxRender's procedural water.

Maybe I should restart that render from scratch, using a modified water surface.

Hmmm....

 

Cheers!