This was the first piece of fan-fiction I've ever read. It was before the release of HBP and there was already a substantial amount of this series written if it wasn't finished outright. I believe the first part was published before OotP was written. Now here's what I remember of what happened in it.

It starts after 4th year when over the summer Hermione sends Harry flirty letters and includes a photo of her in a (black?) bikini. She eventually visits and they get together, but I'm not sure on the details, just that eventually in the series it is revealed that she was being dosed on some sort of love-potion and they split up. At around book 3, Harry gets together with Ginny, but I can't recall if they stay together.

Harry gets buff and tan doing manual labour over the summer for some yard-work company and Draco joins him at it for the next one.

There was also a part with selkies, I think, and also at one point near the end, Harry is forced to accompany Voldemort into the past using a spell, Tempus Fugit, I believe. There, he is forced to save his mother from sacrificing herself, thus preventing Voldemort bein banished.

I didn't finish reading the whole series back then, so I don't know how it ends and that's all the parts I can remember of it right now.
If you recognise which series I'm talking about, but don't know author or name of it, please just try to add whatever you do remember of it. Massive thanks go out to anyone who can find a link for it.

 

Barb's trilogy. The first one is Harry Potter and the Psychic Serpent.

Probably the classic HP fanfic, but I don't think it holds up very well.

 

Wow, that was quick. Thanks a lot!
Yes, I do remember that it dragged on even then, but I want to read it again for nostalgic reasons, I guess.

 

Some google-fu and skim reading the first two chapters and it seems like Harry Potter and the Psychic Serpent by Barb LP could be your fic. Fits some of your requirements, investigate yourself further to see if it's what you were thinking about.


EDIT: nvm. Too slow.