An underwater power generating system comprising a power generating apparatus and a support structure, the power generating apparatus having a mounting portion which is releasably engageable with a support portion of the support structure, the system further comprising a clamp arrangement for clamping the mounting portion to the support portion, the clamp arrangement having an open position in which the mounting portion is removable from the support portion, a closed position in which the mounting portion is fixed relative to the support portion, and a bearing position in which the mounting portion is rotatable relative to the support portion, wherein the clamp arrangement comprises: a clamp member attached to the mounting portion and being rotatable relative to the mounting portion about a first axis in an actuation plane substantially perpendicular to the first axis, the clamp member having a bearing surface and an actuation point, the actuation point being in the actuation plane; an actuation ring arranged for rotation relative to the clamp member about a second axis substantially perpendicular to the first axis; a resilient member attached between the actuation ring and the actuation point of the clamp member; actuation means operable to rotate the actuation ring with respect to the clamp member, wherein, in the open position of the clamp, the actuation ring is positioned such that the resilient member extends in a direction to a first side of the actuation plane of the clamp member, and the clamp member is held by the resilient member in a first position in which the bearing surface does not engage the support portion, and in which the mounting portion is removable from the support portion, wherein, in the closed position of the clamp, the actuation ring is positioned such that the resilient member extends in a direction to a second side of the actuation plane of the clamp member, and the clamp member is held by the resilient member in a second position in which the bearing surface of the clamp member engages the support portion, such that the bearing surface is spaced apart from a reference point on the mounting portion by a first distance, and such that a predetermined clamping force is exerted on the support portion by the bearing surface, wherein, in the bearing position of the clamp, the actuation ring is positioned such that the resilient member extends in a direction to the second side of the actuation plane of the clamp member, and the clamp member is held by the resilient member such that the bearing surface is spaced apart from a reference point on the mounting portion by a second distance greater than the first distance, and such that the mounting portion is rotatable with respect to the support portion, and that the mounting portion is not removable from the support portion, wherein the actuation means are operable to rotate the actuation ring relative to the clamp member, thereby to move the clamp member between the open, closed and bearing positions. a clamp member attached to the mounting portion and being rotatable relative to the mounting portion about a first axis in an actuation plane substantially perpendicular to the first axis, the clamp member having a bearing surface and an actuation point, the actuation point being in the actuation plane; an actuation ring arranged for rotation relative to the clamp member about a second axis substantially perpendicular to the first axis; a resilient member attached between the actuation ring and the actuation point of the clamp member; actuation means operable to rotate the actuation ring with respect to the clamp member,