This paper addresses a filtering problem that arises in the design of dynamic positioning systems for ships and offshore rigs subjected to the influence of sea waves. The vessel`s dynamic model adopted captures the sea state as an uncertain parameter. The proposed adaptive Wave Filtering (WF) consists of a recursive optimization procedure which seeks to identify the dominant wave frequency (the uncertain parameter) by minimizing an appropriate defined performance index. The estimated dominant wave frequency is used to identify the sea condition, based on which adaptive wave filtering (using a Kalman filter) is performed for dynamic positioning purposes. The adaptive WF enables the DP system to operate in different operational conditions and hence, it is a step forward to a so-called all-year marine DP system. The results are experimentally verified by model testing a DP operated ship, the Cybership III, under different sea conditions, in a towing tank equipped with a hydraulic wave maker