Final answer:
During aquatic exercise, resistance to flow is caused by viscosity, which is the measure of a fluid's resistance to flow, resembling internal friction.
Step-by-step explanation:
The phenomenon you're asking about, occurring during aquatic exercise when there is resistance to flow in a liquid, is defined as viscosity. Viscosity is the measure of a fluid's resistance to flow, and it can be thought of as a type of friction between the fluid's molecules. This is why when you move through water during exercise, you feel resistance – each layer of water molecules resists sliding past the adjacent layers, causing a force that opposes your motion, known as viscous drag.