Recipe(tried): Banana Buttermilk Muffins

Here's a muffin recipe I have tried that uses buttermilk. It comes from an
Australian book Fat Fighting Foods:

Banana Buttermilk Muffins

Canola Oil or vegetable oil spray
2cups plain flour
2tsps baking powder
1/2tsp bicarbonate soda
2tbsps raw sugar
1 egg, lightly beaten
1tsp vanilla
3/4cup buttermilk
1/4cup unsalted butter, melted & cooled
1 large ripe banana, mashed
Preheat oven to 400F Spray 1x12 muffin tin with canola oil spray. Sift flour,
baking powder & soda together into a large bowl. Add sugar. In a smaller bowl
mix together egg, vanilla, buttermilk, butter & banana. Add mixture to flour
mixture & mix gently. Spoon into prepared tins. Bake 20-25 minutes, until
well risen & golden brown. Cool on wire rack. Store in an air-tight
container. Makes 12.

I make a lot of muffins & understand where your questions are coming from.
Your first query about the water bath sounds quite feasible as I half fill any
vacant holes(if I'm only making 8) with water as I read that muffins like a
humid oven.