A laboratory experiment is conducted to study the mobility and the segregation of aquatic bidisperse granular columns. The effects of the ambient fluid, the particle composition, and the initial geometry on the dynamics of bidisperse granular columns are investigated. It is identified that the ambient fluid plays diverse roles in the phenomenon but is to retard the collapsing process in an overall sense. The instantaneous frontal positions of the granular mass in a bidisperse collapsing case could be shorter or longer than in a monodisperse case under the aquatic condition, but its final run-out is always longer. Compared to dry cases, particle segregations in aquatic cases are found to be more pronounced with columns composed of fine and coarse particles than with columns composed of fine and medium-sized particles. In general, the segregation phenomenon becomes less obvious at relatively large values of the finer-particle fraction and in cases with relatively small particle size difference. It is demonstrated that particle segregation contributes to an increased mobility of the bidisperse granular mass, probably due to the fact that smaller particles fall downward through the gaps between larger particles as the mixture deforms continuously, leading to an increased possibility for large particles to be separated by isolated small particles or an increased possibility for the rolling friction to take place of the sliding friction between large particles.