ROME — On a sweltering Monday afternoon, a beautiful day for a swim, Mario Messina noticed some holidaymakers teetering dangerously close to the turquoise waters and sounded five short blasts of his whistle.

“Come down!” Mr. Messina said, with an elegant flip of the hand to a woman leaning over the ledge to test the water temperature. “Come down!”

Despite his Ray-Bans, virtuosity with a whistle and sunny Sicilian disposition, Mr. Messina is not a seaside lifeguard but a Roman police officer in a pith helmet. He is entrusted to protect the Trevi Fountain, one of the world’s most cherished and visited monuments, from the scourge of parched, overheated and advancing tourist armies. And now he has reinforcements.

Because summer is here.

Already, the warm weather has brought a menacing whiff of tourists behaving badly. For months, stiff fines failed to stop a fountain-bathing fad in a city blessed with and born of flowing water.