It would definitely be better to leave it in the water. A few simple calculations beforehand and a measuring spoon can get you set for proper dosing just like dosing liquid medicine.
Lets say you have 1g and you want to dissolve in 100ml of water. Then you calculate the concentration like this.
Total mass of stuff / total volume of water. 1000mg / 100mL water = 10 mg/mL. So 1 mL of the water is 10mg of the molly.
Then, if you want a 100mg dose, you take 10mL, or 2 teaspoons (1 teaspoon is 5mL). If you want a 150mg dose, you take 15mL, or 3 teaspoons.
It's going to taste god-awful, but hey, doesn't all medicine?