is very similar to one I took (not on ship but rather booked by the hotel concierge). I found this on the Celebrity web site (didn't know what cruise you were on). Anyway, we enjoyed it very much and I would recommend it.
OK, I just found this one
on the Carnival web page. Again, similar to the above (except this one doesn't give a price).
I had to walk past a gaggle of gondoliers (poetic, no?) to and from my hotel and St Marks so after the upteenth time, on the third day, I negotiated an evening ride for the two of us complete with serenade for a reasonable price (based on the original asking price). However, I had to supply the vino for us.