I work on an offshore oil rig and have to undergo Helicopter Underwater Egress Training (HUET) every 3 to 4 years. They tell us to wait until the "violent motion has stopped." I think it's so you don't get thrown around the cabin. And yeah, it happens really quick and even when you're expecting it and have been training for it, if you panic you won't get out.
The water rushing thing is usually for cars with doors closed. When it stops rushing there isn't much pressure on the door so you can open it. It wouldn't be relevant here with no doors.
You can’t easily overcome the force of water rushing into the cabin, and you don’t want to get tumbled around like a ragdoll, so you wait until the cabin is somewhat settled