Correct me if I am posting on the wrong Stack Exchange. I am looking for a certain game that I'm pretty sure was on PS1. Around 10 years ago, I just watched the intro of the game. It is a story where there is a boy (main character) and a girl in a town near a sea. Then suddenly tragedy strikes. When the boy wakes up, the girl is nowhere to be found and the boy has amnesia. Graphics is similar to Kingdom Hearts and alike.
Could this be Chrono Cross?
It's a PS1 game. Graphics are roughly similar to Kingdom Hearts. There's only two years between the game, but KH1 is on the PS2. Regardless, if this looks similar to KH1, it is likely a late-gen PS1 game.
there is a boy (main character) and a girl in a town near a sea
The boy could be Serge, the main character of the game. He lives in a small fishing village - not quite a town, but it is on the sea. The girl could be his girlfriend, Leena. At the start of the game, he goes hunting for scales to make a necklace for her, and then meets her at the beach.
then suddenly tragedy happened. When the boy woke up, the girl is nowhere to find
This is a bit iffier. A cutscene plays during this scene (until 4:34), which includes flashbacks to Serge being attacked by a panther as a child, and a wave washes over him. He collapses to the floor. He wakes up to an old man finding him on the beach, with Leena not in sight.
and the boy had an amnesia
This doesn't really match at all. I think Serge does have memory loss regarding the panther attack as a child, but I don't think that would have been in the introduction.
He did travel to an alternate timeline in that cutscene, and as a result his memory becomes different from reality, but that's not really amnesia.
If it's not Chrono Cross, the game might have been Galerians.
The beginning of the plot summary from Wikipedia:
Galerians begins with the protagonist, Rion, awaking in a hospital observation room, unable to remember his identity. He hears a girl's voice calling to him in his mind, begging him to come to her rescue, and he decides to search for her. Using psychokinetic abilities to escape his room, Rion fights hospital security and staff desperately and brutally with his newly discovered psychic powers. He finds that human experiments related to unlocking psychic potential are being conducted in the hospital as part of a grander, more mysterious plan known as the "G Project."
A lot of people have been killed, but he is still looking for a missing girl who will help him prevent humanity from being totally destroyed. I don't think there's anything about being near the sea though.
Here are a trailer and the opening video: