To repair a cracked wooden tub (we often supply this advise to customers who own wooden tubs from other makers) use the following guideline.
1) small crack, no leak: just fill the tub and let is rehydrate overnight. As the wood swells back the crack should be completely reabsorbed.
2) small crack with leak: place inside the tub newspaper pages spread over the crack area (be sure not to use a newspaper printed with petroleum based ink). Then fill the tub. The newspaper does not let the water thru but transfers the humidity to the wood. Proceed as 1) above .
3) medium crack with leak: fill the crack in its deepest area with clear silicon caulking and let dry for 24 hours. Then fill the tub and keep the water inside for 1-2 days, The wood should swell back and a part of the silicon has probably been squeezed out.
You can cut the excess of silicon with a cutter.
4) large crack or asymmetrical crack (triangular shaped): in this case the wood not only cracked but warped as well. Even if re-hydrated it will not return to its position completely. In this case you should fill the crack with epoxy putty (we can supply) + sand to make the surface even and make the repair invisible as much as possible.
This crack happens often when the client uses air-conditioning or heating equipment and as a countermeasure keeps some water inside the tub. This precaution is of course very important BUT please understand that it cannot guarantee 100% that also the external side of the wood planks will be sufficiently hydrated.