Cocktail attire.
The ceremony, as well as pre-ceremony drinks and the cocktail hour before the reception, will take place outdoors. Dress accordingly.
In March the temperature is typically fairly mild in St. Petersburg, with daily highs in the mid-70s and lows in the mid-50s.
Late March is Spring Break season in St. Petersburg and surrounding areas. Flights and hotels are likely to book up or become increasingly expensive as time goes on, so booking as early as possible is highly recommended.