Yes, once you complete your booking and receive the booking confirmation email, your activity is considered fully prepaid. This means you will not need to make a payment for the main service on the day of your activity.
Any additional services offered by the activity provider on-site (such as photos/videos, food and drinks or fuel for rentals) and/or mandatory local taxes, may require a separate payment. To see exactly what is included in your prepaid booking, you can always review the original offer page for your activity.
Please note that if you have made a booking request and paid by credit card, your card will only be charged after the activity provider has officially confirmed your booking (for more details on how booking requests work, have a look here).