Many thanks for all that info, Nicholas. The letters from Barrie to Rupert's mother that you unearthed a few years ago pretty much confirmed me in my belief that Michael's death was an accident, not suicide. As you say, Roger Senhouse is not a very reliable or unbiased source - his own love for Michael (beautifully hinted at in his long letter to Nico after Peter's suicide in April 1960*) meant that he was inevitably jealous of Rupert, and perhaps wanted to hold him responsible for Michael's death, albeit subconsciously. But given that Michael and Rupert seemed happy to have found each other, given that their friendship had Barrie's genuine blessing (evidenced in his letters to Rupert's mother), given that he had seemed perfectly happy to everyone else in the weeks leading up to the drowning, one is left with the obvious question, what possible motive could he have had for wanting to kill himself? Or are we to imagine that perhaps Rupert wanted to end it all, and was so in love with Michael that he determined to take him along for the ride?
* To read the letter, search for "Roger Senhouse" in the database.