As far as longitude is concerned, Italy is on the same longitude as the North Pole, like any place on Earth. All longitudes meet at the poles.
As fast as latitude is concerned, the temperatures of a body of water have little to do with latitude. Northern Italy may be on the same latitude as Maine or Washington State but I can assure you that the water in the Mediterranean will be warmer than either place.
Having said that, temperatures of the water in Tuscany in May will be somewhat too cold to swim in (at least for my taste) albeit probably much warmer than the Ocean water of Maine or even here in San Francisco in August.
Given the fact you are in Florence without a car, I recommend Viareggio or Castiglioncello.
The former is a big resort town. The beach is ok, a bit better south of the city, but at least there will be some vibrant life in town (although not like in Summer).
The latter is a smaller quainter beach town. The beach is rockier but the place is prettier.
Either way getting in the water will require some bravery. But if you don’t have problems getting in the water in Maine or Oregon, you can probably bathe in Italy even in February.
Romagna and Marche will offer much of the same, just farther.