Stir together 1 cup marshmallow fluff and 1 1/2 teaspoons water in a small bowl or 2-cup measuring cup, so that it is a thick but pourable texture. Set aside.
Melt 1 lb milk chocolate flavored melting wafers in a microwave-safe bowl in 30-second intervals, stirring with a spatula between each interval, 60-90 seconds total.
Spread the chocolate (reserve 2 Tablespoons to 1/4 cup of the chocolate to drizzle over the top at the end) evenly over the prepared baking sheet, taking care to maker sure there are no thin or thick spots. Drizzle the thinned marshmallow fluff evenly over the chocolate (don't worry if you get the occasional big blob). If desired, gently swirl the fluff throughout the chocolate with a toothpick or wooden skewer.
Sprinkle 1 1/2 cups mini marshmallows and graham cracker pieces all over the chocolate and fluff. If your chocolate hardens before you've added all of your toppings, simply re-melt it by putting the baking pan into a slightly warmed oven for a couple of minutes. Drizzle the bark with the reserved chocolate.
Set the baking pan aside to set. Once it is completely set, lift the baking mat or parchment and transfer the bark to a large cutting board. Carefully remove the baking mat or parchment (never cut on a silicone baking mat) and cut the bark into irregularly shaped pieces in your desired size.
Store the bark pieces in an airtight container or bag with parchment paper between any layers to prevent sticking.
Notes
If you're using milk chocolate-flavored melting wafers or almond bark, you will not need to temper the chocolate. If you're using cut-up milk chocolate, or other real milk chocolate, you can still make this bark without tempering the chocolate, but it should be kept cold to keep it solid.If you want to make this bark with real milk chocolate and keep it at room temperature, the chocolate should be tempered.