Four aquatic plants (i.e., Potamogeton pectinatus L., Potamogeton crispus L., Myriophyllum spicatum L., and Ceratophyllum demersum L.) that commonly grow in European lowland rivers and lakes and exhibit a variety of morphologies and differences in the internal structures of their stem cross sections were selected to investigate the influence of initial conditions on biomechanical tests. A new method of sample testing in wet conditions is proposed and employed using a bench-top testing machine. The obtained biomechanical characteristics, such as breaking strain, force, and stress; Young's modulus; flexural strain; flexural rigidity; and flexural modulus are presented and discussed. Even when fresh specimens were kept in water before testing in air, their biomechanical parameters were sensitive to fast drying. If the turgor pressure was not maintained in dry tests, specimen stiffness was not preserved and the measured biomechanical properties deviated from those observed in wet conditions. Approximately 43% of the three-point bending tests and 20% of the tension tests showed significant differences between dry and wet conditions in all considered plant species. Bending tests for both conditions performed on P. pectinatus L. and P. crispus L. showed the highest differences in flexural rigidity values, reflecting the adaptation of these plants to changing hydraulic conditions. Stem resistance to tension forces in terms of Young's modulus was found to be different for wet and dry conditions only for P. pectinatus L. Overall, it was revealed that plants constantly submerged in water tend to be stiffer than plants exposed to dry conditions.