The Dolphins of Laurentum — Caroline Lawrence

It's October AD 79. The arrival of a ragged man at the Geminus household sets in motion a series of events which take Flavia and her three friends to an opulent villa by the sea at Laurentum, a few miles south of Ostia. Just off the coast is a sunken wreck full of treasure which could be the answer to all their problems. But someone else is after the treasure, too. As the four children try to recover it, they solve the terrible mystery of Lupus's past.