The marine water strider Halovelia septentrionalis Esaki is one of the few insects inhabiting the sea. Along the Japanese coast the population of this species has been decreasing over recent decades and it has now been designated as endangered or nearly endangered by local authorities. It is essential to know its life history in the field so as to develop conservation measures and also to understand its strategy adapting to the sea. We have studied its life history in Misaki on the Miura Peninsular, ca. 35˚N, probably the northernmost locality not only for this species but also for any Halovelia-species, most of which live in warm tropical/subtropical seas. In the inner bays there, adults and nymphs of this species were skating near the rocky shore with overhanging vegetation. They were feeding on insects, esp., marine chironomids, and crustaceans and they had at least 2 generations a year. They appeared to overwinter in the adult stage, presumably staying in some sort of shelter such as an empty oyster shell which was underwater most of the day. Overwintering underwater would be one of the best adaptive strategies to survive the cold season because the seawater temperature was much warmer than the air temperature. In this discussion, we have compared its overwintering strategy to those of two other marine water striders.