Its best to buy them direct from
DCL as the place is very busy. Often there is five or six cruise ships in port and the slots or booked up by the cruise lines, The park may re direct you back to DCL under the joint contract.
With DCL you get a taxi or coach transfer to the park in timed slots and its well organised, and they ensure there isn't over crowding. The swims are all allocated and organized well.
There is a rolling taxi service back to the pier.
I would not turn up on the day, if you do still prefer to book direct, do it in advance as its popular and there will be hundreds of cruises going there each day.