Any good higher-end restaurant (besides, like, a steakhouse) is going to have options for a pescetarian. The only thing making the request hard is narrowing it down from the 1,000 options available.
If it's mid-May you'll want to make a rez now, especially if it's a weekend.
I'm not AS big a fan of Marea as some others - I like it, but found the secondi lacking.
Other Italian options, at the higher end:
Babbo (I don't find them loud, but some do...)
Some non-Italian ideas:
The NoMad (can be on the louder side)