Bonnard met Marthe in 1893 when she was 24. They married and remained together until her death. She became not only his wife but his model. She had a fondness for the bath and some of Bonnard's best works are the Baignoires. Nude in Front of the Mantlepiece is one example.
There is a parallel to Max's wife who, 'when she fell ill, took to extended baths in the afteroon' (p 153).