The marina for the cruise is at the Yacht Club. They will let you park there if you are booked for an Illuminations cruise. Tell the guard at the front gate that you have a cruise booked. They might have a list of names they can check you off of.
Beaches & Cream (at the Beach Club resort, which is connected to the Yacht Club resort) doesn't take ADRs, so you won't get the three hour parking pass for ADR holders. It might be possible to say you are eating at B&C, but it shouldn't matter if you park at the Yacht Club with a cruise booked.
It's also quite a small restaurant and frequently crowded. You might want to give yourself plenty of time to get in and eat before your cruise is scheduled.