It is not a beach wedding. While the wedding is held in Sentosa, the wedding reception is held in an enclosed, air-conditioned venue. We'd recommend that you dress cosily in cocktail fashion, or in smart casual! (Suits & ties, cocktail dresses are welcome!)
That is a negative; the venue and car park are owned by separate entities. The nearest car park is Beach Station Carpark and rates are $1.20 per hour (or part thereof) before 6pm, and $3.00 per entry after 6pm.
Probably just avoid wearing a wedding dress.