There is no option for such charge in the listing. You can charge the extra money via de resolution centre after a booking is confimed. But first it must be made clear to the guest, so in the listing you should mention the facility and the costs.
@Emiel1 gave you good information. I'd suggest that you'd mention the fee in three places: take a picture of the boat parking space and write about the fee in the tag; list it in the Things to Know section; and list it in the House Rules. Also, you may want to mention it in your reservation greeting message. If the guest says they want to pay for parking, send them a payment request the resolution center.