I usually choose the bags for the specific conditions I will be encountering. Waterproof with rain hood if wet, and oil/cloth canvas when dry. If the bags get wet i just hang them to dry with everything out of it, and all pockets open, and hanging.
I also like to pack gear in ziplock bags to combat humidity with cameras that are not weather sealed, also within the ziplocks I toss silica gels.
But I think in situations such as the tropics, you really cant do much about it.