If you want an association with water (but not coastal) then maybe water vole, water shrew, frog, toad or newt? Mink or coypu may be an option though not true natives. Non aquatic visitors to water could be fox, mole, polecat (depending where in the country your setting is), rat or weasel. Guess it depends if the essense of the animal is more important than the physical characteristics (thinking of the seal's ability to be at home in the water and dry land, bridging the gap between the two elements). Do you have any scope for artistic license?