It is called a ventral fin. It enhances lateral stability due to the pendullem effect caused by the floats. If you look at the original Beaver configuration, it had a ventral fin. Kenmore Air Seattle STCd the the small finlets attached to horizontal stabilizer. These tend not to get smashed and bent around a high dock. Some aircraft such as the Cessna Caravan have finlets, and an enlarged dorsal fin and rudder.