A possible explanation is that water can help the solubilization of the base (OH-) and this favor the formation of the reactive OXO-palladium intermediate that has been proposed in the transmetallation step (Org. Lett. 2006, 8, 4071; J. Org. Chem. 1998, 63, 461; Angew. Chem. Int. Ed. 2013, 52, 7362).
Any Luck ??? I found this one -- New Catalysts for Suzuki-Miyaura Coupling Reactions of Heteroatom-Substituted Heteroaryl Chlorides A. S. Guram, X. Wang, E. E. Bunel, M. M. Faul, R. D. Larsen, M. J. Martinelli, J. Org. Chem., 2007, 72, 5104-5112.