In a large bowl, stir together the water and gelatin until the gelatin is dissolved.
Drain the pineapple, reserving the juice.
Set the pineapple aside.
Add enough water to the juice to make 1 cup, and stir it into the gelatin mixture.
Gradually stir in the yogurt and ricotta cheese.
Chill in the refrigerator about 30 minutes or until thickened but not set.
Fold the pineapple, marshmallows into the gelatin mixture.
Transfer to a 1-quart mold.
Chill in the refrigerator about 2 hours or until firm.
Unmold the gelatin onto a serving plate and serve.