Ciao Golden,
I think Monte Argentario would be perfect for scuba divers and snorkelers, particularly its proximity to Giannutri and Giglio - as would be Elba. But considering your time restraints, definitely Monte Argentario is closer and faster to get to and doable from Siena.
Monte Argentario tends to attract the "rich and famous" type of vacationer so if you want to stay on the promontory, I'd definitely also head to really nice places such as these two:
Baia d'Argento
Torre di Cala Piccola
Of course, you didn't say much about what you're looking for, there are definitely other options as well.... and you could stay off the promontory as well, drive to Porto Santo Stefano for example would not be long if you stayed in Talamone, for example.
As far as diving, Porto Santo Stefano would be the good place to head to - several dividing centers will offer gear rental as well as set excursions. This is just one as an example:
http://www.argentarioscubapoint.it/
You can pick up a few more suggestions/links from an old thread here on our forum - both the Argentario and Piombino area are popular for dives:
http://www.discovertuscany.com/forum...g-tuscany.html
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