The approaches to locomotion in liquid environments found in nature are very diverse. However, a common theme is the use of soft and flexible fins or appendages for momentum transfer. Unlike locomotion on land, where discrete point forces can reliably be used to model the reaction forces needed to transfer momentum, the pressure fields that provide the reaction forces needed for locomotion underwater are more complex. Simple models are needed to estimate underwater locomotive forces and enable real-time control of bio-inspired fin propelled vehicles.