If you ate a whole bag of marshmallows would you float better in a swimming pool?

No.

The reason marshmallows float in water is because they have a very small mass compared to their volume (they have a low density).

If you eat marshmallows, all you're doing is adding their mass to your own. You won't gain any additional volume. Instead of adding volume to the outside of your body, you are removing space from inside your body.

All you're going to do by eating marshmallows is increase your mass by a very small amount, which, if anything, will actually cause you to sink easier than before.