1.  The paper clearly stated:  Mixing of the products (i.e. 2 different dilbit mixtures) with seawater led to water uptake that increased the density of the resulting mixture, however the mixtures remained buoyant in all cases.
2. The paper also said sediment and wave action caused the mixture to disperse as floating tar balls.
3. The paper also indicated that the vast majority of a 2007 leak in Burrard inlet was recovered.   And 80% of the spill in 2010 Kalamazoo in a flooded river was recovered and the remainder formed stable mineral aggregates.  (i.e. the aggregates can be recovered with new technologies)
Finally, hate to bust your horn but there is far more chance of a ferry having an accident in BC than an oil tanker.