We were there today, but did not do the Seaventure. The cost was an additional $59/pp and yes it was open. We were told we could book it at registration.
Still technically a soft opening. You can buy it once you're in the park for the day, $60/person and limited openings. Over time, it will be something you reserve when you make a reservation, but for right now, you can book when you check in, or later in the day if there is still room, at the guest service desk.
Had a nice chat on the phone with the UK Seaworld operative, she was brilliant and very helpful! She gave me a phone number (freephone from the UK I'm told) to phone and book SeaVenture -
0080033441818.
I phoned and booked 2 SeaVenture experiences with no problems. On arrival to Discovery Cove they will give you your time slot.