While the freeways would probably be quickest (route 105 east to route 405 north to route 10 west) even with traffic, you could take surface streets and still get there in under forty minutes.
To get from your hotel (the Double Tree) in El Segundo to Main
Street in Santa Monica, avoiding freeways and staying as close to the ocean as possible, I would:
1. From the hotel, take Grand Avenue west (towards the ocean) until it ends at Vista del Mar.
2. Turn right (head north) along Vista del Mar until it curves and turns into Culver Boulevard in Playa del Rey. The ocean will be off to your left.
3. Soon after crossing the Ballona creek, take the circular turn off Culver Boulevard onto Lincoln Bouelvard and head north on Lincoln Boulevard.
4. Take Lincoln Boulevard north until you reach Washington Boulevard. Turn left and head west again along Washington Boulevard. (This area isn't that scenic, really.)
5. Veer right off of Washington Boulevard onto Abbot Kinney Boulevard. This will take you through an area of Venice Beach with furniture shops and restaurants, not the beachside "boardwalk" area of Venice Beach.
6. At the end of Abbot Kinney Boulevard is Main Street. Travel north on Main Street. The portion of Main Street that's in Santa Monica proper is just north of Rose Avenue.
There's a few short cuts you could take to make the trip a little quicker and perhaps a little less scenic. The directions above are about the best combination of scenic and simplicity that I can think of. I would esitmate that it should take no more than forty minutes to get there. Since it's a wedding, I'd make it an hour just to be safe, since I'm not sure where along Main Street in Santa Monica you are headed to.