Basically they are just the same. Group of boys, who joined a certain sport half heartedly, but ended up being very serious on doing it.
Both are comedy(but waterboys is funnier IMO)
Both have sexy teacher
Both shows the beauty of friendship.
A very inspiring story if you are into sports.
Both movies focus on an "unlikely" sports activity and shenanigans around it.
Both movies feature a team, but in Ping Pong supporting characters are more in the background.
In both movies Naoto Takenata plays the mentor figure.
Both movies underline the value of friendship.
«Waterboys» (2001) is somehow the prequel to this drama, even though you can watch one another without having watched the other one
Its from the same director with a similar story (zero-to-hero) and feelings : some loser guys discover synchronized swimming and pursue through hard work their new dream.