Posidonia oceanica meadows are fundamental ecosystems for Mediterranean marine biodiversity. In recent decades, a drastic decline of this habitat has been observed, mainly as a consequence of increasing anthropogenic pressures. Therefore, several restoration projects have been launched using different techniques and materials. Although many restoration interventions resulted successful in ensuring the stability of transplanted cuttings, only a limited number of current techniques rely on eco-compatible and non-persistent materials. For example, one widely used technique involves concrete frames and metal nets, which can persist in the marine environment for many years without degrading. To reduce the environmental impact of anchoring techniques, this study tested alternative materials capable of degrading within 1–2 years after deployment at sea. These materials should provide the stability needed during the initial anchoring phase, without persisting beyond the required time. The behaviour of gypsum and hydrated lime putty mixtures in seawater was therefore evaluated to identify the ideal compositions in terms of dissolution. Cubic modules of 5 cm³ were produced using four different compositions: A, 100% gypsum; B, 50% gypsum and 50% hydrated lime putty; C, 67% gypsum and 33% hydrated lime putty; and D, 33% gypsum and 67% hydrated lime putty. Mass variation and dissolution rate were monitored both under controlled conditions and in the field. In both contexts, mixture B, followed by mixture C, showed more stability over time, while mixture A was the least resistant. Mixture D showed a peculiar pattern, with an increase in mass, under controlled conditions. Based on these results, mixtures B and D were selected to produce transplantation modules that will be tested at sea to verify their real effectiveness. Although this study represents a preliminary experimental phase, further tests on these materials could support the development of lower-impact and lower-cost modules for P. oceanica restoration.