I thought the same thing too, Simon. But looking over the judges list messages, and reviewing the rulings for Magic Reflector, they all seem to say the same thing. And that's when a card is destroyed because its time has expired or when its maintenance cost is no longer being paid or when a game mechanic destroys it, then those are the only times a "Reflector" Counter won't protect it. Because none of those are an effect.
But look at the ruling for Union Monsters:
If you use "Magic Reflector" to place a counter on a Union Monster that is an Equip Card, and the Union Monster would be destroyed instead of the equipped monster, remove the "Magic Reflector" counter from the Union Monster instead, and the Union Monster is not destroyed.
And a Union Monster's self-destruction effect is no different from from either of Heart of Clear Water's self-destruction effects. In fact, I think the fundamental difference here is that Heart of Clear Water is an actual effect, not an expiration or a time out like Swords of Revealing Light, nor failure to pay a maintenance cost like Messenger of Piece, nor a game mechanic like activating a second Field Spell Card with one already active.
If Union Monster can benefit from the counter, then Heart of Clear Water should be no different.