When I drive through water (not necessarily deep), sometimes the headlights dim a bit for a second or two.

Is this because the auxiliary drive belt gets wet?

Hi Matty,
When this happens, are you by chance using the windscreen washers at the time? Another member of the failed FL2 clutch/DMF club, twice.

The windscreen washers are always on auto, and sometimes the water that causes the lights to dim is deep enough to splash onto the screen, and other times not.

Wipers not Washers

Yes, wipers

If you operate the windscreen washers with the headlamps switched on, the water from the headlamp power wash will affect the beam.
Is this what is happening? Another member of the failed FL2 clutch/DMF club, twice.

Ah, I get you now. No, thats not whats happening, the lights definitely dim for a few seconds.

My first thought was that water had got onto the alternator belt, causing it to slip, although I didn't hear any belt squeal

This has come up before, and is usually agreed to be water thrown up by the wheels on to the lights. Almost certainly not the belt, I'd say.

Agree with pab; not caused by the belt slipping  Another member of the failed FL2 clutch/DMF club, twice.

Just so Im clear, the water is thrown up onto the lens, and this momentarily causes a reduction in light transmission?

That's right. But, of course, I can't say for sure that's what's happening in this particular case.

I have noticed a similar effect recently as the weather seems to have been a bit damp! Emerging out of deeper puddles I have only noticed when the light becomes brighter. I just put it down to the water affecting the lens focussing or somefink like that.

Of course, if the water is spraying up onto the lights you're going far too fast through the water, which is something I would never do!

but its sooooo pretty!

A country car wash, I call that. Certainly mine came out cleaner than it went in.