I went to the regatta last year. To be honest it’s boring, long gaps of time to see boats pass. But I can add it to my “been there, done that” list.
Unless you pay big bucks for the few reserved seats, you stand alongside the Grand Canal. I stood across the canal from the grandstand. Local police will make you move if you sit on the dock steps. After watching the parade of boats and a couple races which took a couple hours, most people left to get gelato.
I met very nice people while standing alongside the canal, a French couple from Toulouse and an Australian couple.
The Blessing of the Boats at Basilica di Santa Maria Della Salute was less interesting.
There’s enough to do and see in Venice that you or your RS guide will have no problem rearranging schedules. FYI, I’m returning to Venice next year for my 4th visit to my favorite city.