Fan belt slipping problem

Went through a bit of water the other day in my Series, nothing serious about up to the floor pan. Coming out the other side the charging light came on. Went off after a minute, presumably when it dried out. Looks like the fan belt slipped when it got wet. The belt is tight and don't want to tighten it any more for fear of buggering up the alternator bearings Any one got any ideas how to stop slipping
You used to get a product called "belt dressing" which may cure it but I can't find any.

no-nails?

Try a new belt first.

Thanks Andy. It looks new but may have lost a bit of grip.
Tim.