I went to TripAdvisor to see who the top-rated water taxi companies are in Venice. The top-rated outfit is Venice Quality Transfers. I read some of their reviews, and they seem to be consistently rated excellent. Also, try Consorzio Motoscafi or Venice Water Taxi. Both are rated highly, but reviews are not as consistently good as VQT. The other option, which would save you some money would be to take the Alilaguna Water Bus. I say this because it stops at Piazza San Marco and you are just a few minutes walk from the stop. The downside would be that it is not private and they can get crowded. If you definitely want a private water taxi, I would contact Venice Quality Transfers.