In the beginning of May the sea in Lazio will be very invigorating. You can swim, but you can not swim, unless you are a "walrus". I recommend the resort town of Terracina, 100 km south of Rome. There is a good infrastructure. Many hotels with swimming pools.

If you are not traveling for a couple / three nights, you can fly to the south of Sicily or Sardinia. There the water will be a couple of degrees warmer. Vacationers at this time usually bathe in swimming pools, sunbathe

company The sea is located 30 minutes from Rome, station. The coast is 30 km from Rome. You can swim in Anzio, Terracina, Lido di Ostia and in many other towns on the coast.