Benedictolandia

philosopher's stone: no baths/showers. mentions a cozy fire feeling like a hot bath so must have bathed at least once in his life. wipes out a troll in the bathroom so that's something. 2/10.

chamber of secrets: the dursleys let him out to use the bathroom twice a day, we can assume he cleanses occasionally here. spends most of the book in a bathroom doing many things except bathing. 5/10.

prisoner of azkaban: no baths. a filthy year for the young wizard. 0/10.

goblet of fire: one of the tents at the quidditch world cup has a birdbath. perhaps harry bathed in that but it isn't specified. has a bath! a bubble bath! an important occasion. 9/10.

order of the phoenix: possibly bathes at the dursleys. droplets of grawp's blood are said to shower upon him. probably the closest he came to bathing that year. 4/10.

half blood prince: no baths. poor ginny. 0/10.