[Kamonichan]

Spoiler for my take on the ending:

If you want to take it all in, Haru's text message to their friends may have been sent as a farewell-suicide note, meaning he somehow either knew or intended he and Sora would die, either at the lake or some time later. Or he could have been referring to the two of them being taken in by their aunt and uncle, mistakenly thinking they would be happy that way and unfortunately drowning before that happened. Or after saving themselves, he sent it as a farewell-goodbye message, letting them know he and Sora were all right and were going to make their love work somehow.

As for them sinking into the lake, you can either say they actually drowned, or Nao, who was also looking for Sora, came to the same conclusion as Haru--albeit slightly later--saw them sink below the surface, and dove in to save them. Sora may have told her to leave them after that, or Nao might have felt they needed to be alone and left them when she knew they were all right. That would add to her opinion at the end that she couldn't blame them for their feelings and wanted to support them. The second ED "suggests" that she might have saved them, when Haru and Sora (chibi form) are saved by Nao (also chibi form). Of course, obviously that segment of the ending credits can't really be called canon, but it offers an explanation.

I think either interpretation works. I prefer to think of it as both, really. In one sense they "died" and in the same sense they kept "living." Either they figuratively die by leaving their old world (Japan and its customs) behind and go on to live in Europe or some place (arguably a "paradise" or "afterlife" where they are accepted), or they literally die (drown) and continue their "lives" (and their love) in whatever place their souls go (heaven, Nirvana, etc.). That lake was supposed to change who they were, so either way, them going into the lake and coming out is a sort of rebirth for the two of them.

And yes, 11 out of 10 for this show.