31/07/1980 - Harry is born
31/10/1981 - Lily & James are murdered / Vodemort is cast from his body
01/11/1981 - Harry is left on the Dursley's doorstep at night
02/11/1981 - The Dursley's find Harry(in the morning), Vernon and Petunia discuss what to do about it, and decide to leave Harry somewhere where he cant be traced back to them.
14/09/1991 - Harry receives his Hogwarts letter and retuns it, writing that he doesn't understand what it says in mermish
01/09/1992 - Harry starts his third year at Hogwarts, under a different name
The Dursley's drive out to the ocean, and hang around until the middle of the night. They find a small boat and put Harry in it, then leave. Tide comes in and takes the boat out to sea.
A group of traveling merpeople find him and bring him with them. They use mermagic to give him the ability to live underwater, and to understand them. It was something meant for temporary use only, so the longer he used it the more it changed his body to more resemble those who made it, the Sirens of Greece. He got his Hogwarts letter mid September, and replies to it, in mermish.
Albus go to see Harry himself, and takes him to a wizard friend of his, Nicolas Flamel. Nicolas teaches Harry correct gramar, normal human things, and magic. Harry has a problem with clothing, pants in particular, and cross dresses for a while. He excels at magic and gets up to third year.
The next year, Harry attends Hogwarts under a different name and enters third year.
Yeah, that's the gist of it. Here are a couple of things that didn't really fit in at the top. They're in a random non-order.
1) Harry knows some English, from listening to tourists, he's just not very good at putting sentences together, but he can understand it fine, that's why he can get so far in his magic studies.
2) The Dursley's didn't mean for the tide to take Harry away, they just wanted to leave him where someone else would have to take care of him.
3) The changes that happen to Harry's body are that his hair changes to a deep green-blue with lighter blue-green highlights to act as camouflage. His eyes take on an iridescent quality, and his skin becomes pale, to blend in with the sand.
4) I figure that merpeople probably have magic of their own. The breathe-underwater/translator thing is a choker that is woven around the persons neck so that it can't fall off. Harry had to have it rewoven around his neck many times. Each time he grew.
If there's anything that doesn't make sense, other than the entire idea of it, tell me, and I'll add something about it.