Countless Royal Caribbean passengers may have had their privacy profoundly violated by an ex-employee now serving 3 decades behind bars for child porn ... an attorney for the culprit's victims now claims.

Arvin Joseph Mirasol, a former cabin attendant for the cruise Mirasol, was sentenced to 30 years in federal prison back in August after pleading guilty to filming a number of minor girls -- some as young as 2 -- while they were undressing on the Symphony of the Seas cruise ships he worked on.

In a class action lawsuit, he's now accused of filming hundreds of passengers without their knowledge ... after planting hidden cameras in bathrooms.

He was first detained in February after one young female passenger found a camera hidden beneath her sink on one Royal Caribbean cruise that had departed from Florida. When the ship returned to port, Mirasol was arrested. Investigators confiscated his devices, which included numerous videos of children getting undressed and using the shower.

Per investigators, Mirasol also confessed to spying and recording children while hiding under beds in cabins.

A passenger had filed an earlier lawsuit which alleges nearly 1,000 people may have been captured on Mirasol's hidden cameras ... and some of the footage was actually posted online.

We've reached out to Royal Caribbean for comment ... so far, no word back.