Frog Suit

From the Super Mario Wiki

A Frog Suit is an item found exclusively in Super Mario Bros. 3. The Frog Suit transforms Mario or Luigi into Frog Mario or Frog Luigi. It grants Mario or Luigi the power to jump higher, swim faster, resist water currents better, and hop while walking on the ground. When Frog Mario or Frog Luigi is hit by an enemy, they revert back to Super Mario.

The Frog Suit is a handy item to use in underwater levels. However, Frog Suits don't provide much help on dry land. When out of water, Frog Mario and Frog Luigi move very slowly, as they can only move by hopping (instead of running). On the positive side, Frog Suits do allow the Mario Brothers to jump higher than they can normally.