Taku travels to his hometown for his high school reunion. During the trip, he recalls the memories of the days in high school. Friendship, subtle love, a trip to Tokyo and so on, all came back to him as the film evolves.
Ocean Waves was made by Studio Ghibli (Spirited Away) and also was the first film directed by someone other than Hayao Miyazaki. Instead, Ghibli wanted only the youngest staff members to make a film with a smaller budget than before (though it ended up going over both budget and schedule). Still, what we got was a more mature film involving a love triangle, and reminiscing about old friends.
In the same way Taku remembers his school days, maybe you will too think back at people you've met, and friends you'll never meet again.