Spoiler - Ending? Really?

Why did they think it was suicide? As far as the facts go, he just slipped and banged his head and drowned.

The viewer knows he was distraught, and may have gone to the dock to kill himself, but no one in the film knew about his newly discovered dark secret.

He had no apparent motive, only Meredith Potter (Hugh Grant) seemed fussed by his relationship with underage Stella. He didn't hang or shoot himself, so there was no obvious suicide method.

So why would they assume he killed himself, rather than accidental death?