A wave of raids and arrests has targeted opposition figures and government critics.
The boats, carrying more than 80 people total, had been attempting to cross to Italy.
The coastguard rescued 17 others, two of whom are in critical condition, says Sfax court judge Faouzi Masmousdi.
Jaouhar Ben Mbarek, a critic of the president, is a prominent member of the main opposition coalition.
The arrests come amid a continuing crackdown on critics of President Kais Saied.
Rights activists criticise presidents comments, say they are intended to create an imaginary enemy.
In Sunday?s second round, voters are choosing among 262 candidates seeking to fill the 131 remaining seats.
Opposition had raised concerns that Saied was ill after the president didnt appear for almost two weeks.
Ghannouchi, speaker of dissolved parliament, told to appear at police station after arrests of presidents critics.
Il nuovo business degli scafisti. Mentre il regime di Saied ha spento la primavera