The European Otter (Lutra lutra), also known as the Eurasian otter, Eurasian river otter, common otter and Old World otter, is a European and Asian member of the Lutrinae or otter subfamily, and is typical of freshwater otters.
The European Otter is the most widely distributed otter species, its range including parts of Asia and Africa as well as being spread across Europe.
European Otters are strongly territorial, living alone for the most part. An individual's territory may vary between about one and forty kilometres long with about 18 km being usual.The length of the territory depends on the density of food available and the width of the water suitable for hunting.
Males and females will breed at any time of the year, and mating takes place in water. After a gestation period of about 63 days, one to four pups are born, which remain dependent on the mother for a year. The male plays no direct role in parental care, although the territory of a female with her cubs is usually entirely within that of the male.
en.wikipedia.org/wiki/European_Otter - 5.10.2010
Looking at the European Otter, you are sure that it is an excellent swimmer.
Furthermore it is able to dive up to 8 minutes and up to 18 meters depth.