Hi, thought I should share here my experience earlier this week. I was travelling in Vietnam and touring Mekong river. As part of the trip, there is this 'Mekong river' boat ride. Well, it is actually 'sampang' that can take 4 passengers and 1 rower. As I was boarding this sampang, it starts to rain but very light. Once out in the river, it starts to pour like crazy!
It is either you jump into the river or just sit tight and wait for the sampang to row up the river to the bigger boat waiting upstream. Luckily I have always kept a spare rain coat inside my camera sling bag. I only have enough time to pull it over me to cover my bag. I cling my camera under my chest with the rain coat covering it. That 15 minutes is just like hours and I am totally soak with no sign of the rain stopping any sooner.
So the lesson I learnt here is, if not for my spare rain coat, my bag of lenses and camera will need to go for water proofing test. So, do keep a spare rain coat inside your bag.