Yellow Submarine (with fillable balast tank!)

Simple submarine boat made on demand of my children :)
Balast tank is operated by syringe (or, as my younger child prefers - just by mouth) through a thin elastic hose (it would be good to find something more elastic than this used by me).
Balast - 4cm of threaded rod M8, notched at the end for screwdriver. You can trim (more less) this boat by moving this rod along of keel.
For easy connecting hose, into boat is glued part of thin pen cartridge, I glued it by hot glue, there is a place to fi