hi, if you are in italy
, ok, you might have lots of options.
where? north? south? centre?
if north, end of september, beginning of october...in trieste (NE corner of italy) there is barcolana, a huge event with more than 2000 boats (think is first weekend in october, you can check their website www.barcolana.it):
it's something incredible, and there you can be hosted as crew in some boat and participating the regatta
a week later in venice there is the "smaller sister" veleziana, also you can get in touch with organization and find out if someone can host you.
if you want to sail by yourself you can find something in chioggia, a "small venice" maybe 20nm south of venice, i'm not sure but maybe OLTREMARE has some boats to rent. from there you can reach venice or via the laguna (channels) or via the sea, anyway stunning. swimming: water is not beautiful and temperature might be not so friendly, but getting to venice by boat is priceless.
down on east coast
is not so interesting until south (bari, trani and even more south like otranto and leuca) and actually is an area of which i dont have knowledge.
west/south is sicily
: eolian islands are a must, and portorosa hosts a lot of charters (turistica il gabbiano, for example), then going north could be nice spend days in pontine islands or costiera amalfitana.
further north for a couple of days elba island and capraia.
further north is liguria and cinque terre (portofino....)
it depends how many days you want to spend at sea.
2 to 3 days think regattas/venice/elba island you can do and is possible find a boat.
more days (1 week) the choice is wider.