An underwater structure includes a power generation unit, which includes a main body, a mounting portion which extends from the main body and which defines a mounting axis, and a support structure adapted for engagement with a bed of a body of water, and support housing. The mounting portion defines a substantially continuous mounting surface which extends substantially completely around the mounting portion, and the support housing defines a substantially continuous support surface which extends substantially completely around the support housing. The mounting surface and support surface are arranged to abut one another substantially continuously when the power generation unit is mounted on the support structure. The mounting portion and the support housing are adapted to cooperate with one another for mounting of the power generation unit on the support structure in any polar orientation about the mounting axis.
