An underwater structure comprising: a power generation apparatus including a main body, a mounting portion rigidly connected to the main body and defining a mounting axis, a connection carrier, a connector mounted on the connection carrier, and an actuation mechanism in communication with the connection carrier; and a support structure adapted for engagement with a bed of a body of water, and including a support housing, a support connection carrier received within the support housing, and a support connector mounted on the support connection carrier; wherein the mounting portion and the support housing are adapted to cooperate with one another for mounting of the power generation apparatus on the support structure; and in which the connection carrier is releasably engageable with the support connection carrier, such that the connection carrier is moveable between: an engaged position in which the actuation mechanism is operable to rotate the mounting portion about the mounting axis; and a disengaged position in which the actuation mechanism is operable to rotate the connection carrier about the mounting axis, characterised in that: the mounting portion defines a substantially continuous mounting surface which extends substantially completely around the mounting portion, and wherein the support housing defines a substantially continuous support surface which extends substantially completely around the support housing, the mounting surface and support surface being arranged to abut one another substantially continuously when the power generation apparatus is mounted on the support structure, and wherein the connection carrier is moveable relative to the power generating apparatus in a direction extending substantially parallel to the mounting axis. a power generation apparatus including a main body, a mounting portion rigidly connected to the main body and defining a mounting axis, a connection carrier, a connector mounted on the connection carrier, and an actuation mechanism in communication with the connection carrier; and a support structure adapted for engagement with a bed of a body of water, and including a support housing, a support connection carrier received within the support housing, and a support connector mounted on the support connection carrier; wherein the mounting portion and the support housing are adapted to cooperate with one another for mounting of the power generation apparatus on the support structure; and in which the connection carrier is releasably engageable with the support connection carrier, such that the connection carrier is moveable between: an engaged position in which the actuation mechanism is operable to rotate the mounting portion about the mounting axis; and a disengaged position in which the actuation mechanism is operable to rotate the connection carrier about the mounting axis, an engaged position in which the actuation mechanism is operable to rotate the mounting portion about the mounting axis; and a disengaged position in which the actuation mechanism is operable to rotate the connection carrier about the mounting axis, the mounting portion defines a substantially continuous mounting surface which extends substantially completely around the mounting portion, and wherein the support housing defines a substantially continuous support surface which extends substantially completely around the support housing, the mounting surface and support surface being arranged to abut one another substantially continuously when the power generation apparatus is mounted on the support structure, and wherein the connection carrier is moveable relative to the power generating apparatus in a direction extending substantially parallel to the mounting axis.