A mounting system for a water current power generating system which includes a support structure for location on the bed of a body of water, and a power generating apparatus including rotor assembly, an electrical power generating unit rotatable by the rotor assembly and operable to output electrical power, and an electrical output for export of electrical power from the power generating apparatus, the mounting system comprising: a transition piece having a first engagement surface and a second support surface spaced apart from the engagement surface, the transition piece being mountable on such a support structure such that the first engagement surface abuts a first support surface of the support structure, and the second support surface being arranged for receiving a power generating apparatus thereon; a first clamp mechanism for releasably attaching the transition piece to such a support structure; a second clamp mechanism for releasably attaching the power generating apparatus to the second support surface; a connection means to which an electrical output of the power generating apparatus is releasably electrically connectable; and an export connector electrically connected with the connection means and to which a power export cable (44, 53) of the system is releasably electrically connectable; wherein the transition piece further comprises a side wall extending between the first engagement surface and the second support surface; and either: the side wall comprises an aperture therethrough, and the export cable extends through the aperture; or the export connector is located in a side wall of the transition piece, and provides an internal connection which is electrically connected with the connection means, and an external connection to which a power export cable of the system is releasably electrically connectable. a transition piece having a first engagement surface and a second support surface spaced apart from the engagement surface, the transition piece being mountable on such a support structure such that the first engagement surface abuts a first support surface of the support structure, and the second support surface being arranged for receiving a power generating apparatus thereon; a first clamp mechanism for releasably attaching the transition piece to such a support structure; a second clamp mechanism for releasably attaching the power generating apparatus to the second support surface; a connection means to which an electrical output of the power generating apparatus is releasably electrically connectable; and an export connector electrically connected with the connection means and to which a power export cable (44, 53) of the system is releasably electrically connectable; wherein the transition piece further comprises a side wall extending between the first engagement surface and the second support surface; and either: the side wall comprises an aperture therethrough, and the export cable extends through the aperture; or the export connector is located in a side wall of the transition piece, and provides an internal connection which is electrically connected with the connection means, and an external connection to which a power export cable of the system is releasably electrically connectable. the side wall comprises an aperture therethrough, and the export cable extends through the aperture; or the export connector is located in a side wall of the transition piece, and provides an internal connection which is electrically connected with the connection means, and an external connection to which a power export cable of the system is releasably electrically connectable.