After Onyx finishes his bath, the tub is seen to be empty of water, although nobody drained it and since it is sitting in the middle of the hotel room floor, it obviously is not attached to any drainage pipe.
Onyx is taking a bath, with soapy water up to his waist in the tub. However, as he moves the swimsuit he is wearing can be seen over the suds.
When Martin Onyx is showing signs of a high fever and complaining of chills, Duke tells Flint he once saw a man with a fever of 108 degrees. Medical thermometers were not in common use in the United States until the 20th century.