The objective of this paper is to study the effectiveness of an opportunistic condition-based maintenance strategy for the sea water pumps in a nuclear power plant. For the presented study a primary seawater system consisting of four sea water pumps, supplied by three water intake chambers, have been considered. There exists functional dependency and state interaction between these pumps due to the operation policy. This results in the degradation of the pumps at different rates. In order to ensure the availability of the pumps for the effective operation of the plant, an optimal maintenance strategy is required. To that end, both functional dependency between the pumps and state interactions between the components of the pumps are firstly modelled. An opportunistic condition-based maintenance strategy considering these dependencies is then proposed. The proposed opportunistic maintenance policy allows consideration of the quality of the maintenance actions. A cost model is then developed to evaluate the performance of the proposed opportunistic maintenance policy. Long term simulation results show the effectiveness of the proposed model and opportunistic maintenance policy.