You can easily bus to and from Penzance from Marazion using the regular First bus services that stop all along the main road. It's only 85p and less than fifteen minutes to get to Penzance from Marazion, or you can just walk the route along the sands of Mount's Bay. That'll take you about an hour, if I remember correctly, but it's worth doing at least once.
You can easily travel by the "300" Service Bus from Penzance to Marazion. You will travel only 10-15 minutes and if you choose this bus you can see everything from the roofless top floor. You can catch this bus from every two ours. Although you can take another bus but it won't be so nice. The price for a return ticket is ₤1.40 (from Penzance to Marazion).