Liked the Mountebank Dentist very much. On closer inspection I found, however, that the inscription's second line is Chi non sa navigar and not ... fa ... . You will notice that the wavy vertical line has no cross bar as does the word fondo in the line below. In writing the word sa the long-s is used as at the beginning of a word, in contrast to the round-s which comes at the end of words. If you look up an English text written in Gothic script (as used in Italy since the 15th c.) you will find the long-s letter used everywhere, see for instance this page from a bible
http://en.wikipedia.org/wiki/File:Piers_plowman_drolleries.gif .

The verb 'sapere' instead of 'fare' also makes more sense in the translation:

S/he who knows not how to swim goes to the bottom