Seems there must be a specific Tasmanian requirement for the pilot to possess a Restricted Coxwain's certificate. Either that or the pilot is required to drive a commercial vessel as part of his or her responsibilities. Good luck getting that ticket in a hurry, you'd be up for near 6 months solid on a boat !
The Coxwain's certificate is an interesting set-up. Conceivably you could fly an A380 full of passengers as F/O or almost Captain in the same amount of hours that it would take to get a Coxwain's certificate to drive a 5 meter runabout up a quiet river with paying passengers onboard! (1500 hours)