A particularly appealing entry in Ryder's ``Just for a Day'' series, evoking the experience of being a sea otter. The child addressed in the second person here is a girl, seen only on the first and last pages, who spends a day as an otter. The rest of the book describes the otter, dozing on a wave on a misty morning, diving through kelp for food, grooming (a note explains the importance of this for retaining warmth, which is why oil spills are life-threatening), sporting playfully with a frisky pup. Ryder's text is quietly melodious, gently conveying the otter's sensations while examining its behavior. Rothman brings it all vividly to life in full-bleed paintings that capture the shimmering sweep of the sea and the swaying underwater world in delectable blues and greens, with the lively, appealing otter and its companions as the center of interest. Unusually attractive. (Picture book. 4-9)