Can you jus walk up an pay to go in

Yes, but like a fellow reviewer stated, you do save some money if you pre-order online! It's a great place to visit.

Yes. It's just like a normal aquarium. It's often cheaper and quicker to get in of you prepay online.

Yep

Yes. Check for the busiest times beforehand. Sometimes the wait can be long. You can prepay and it helps expedite your entry

10 min walk is something small just for the kids, is not sea world of San Diego Ca ok...