Recently booked with Sealink, and as it is such a generic code, I presume it is available for the use of anyone. 10% works out to be quite a good discount when you start booking for entire families, saving upwards of $20.
The email from Sealink read as follows; As a thank you, we would like to provide you with a discount code which will give you a 10% discount for all SeaLink Ferries and 10% off any Captain Cook Cruises Product. Please use code: REPEAT when booking in the future.
Used these guys recently to Rottnest Island. They were quite good.