Climate change, altering the precipitation patterns, is intensifying soil degradation due to the reduction of soil moisture, posing serious risks to global food production. In this framework, the present research aimed to evaluate if the addition of polymers, obtained by marine organisms, improved water retention in degraded soils and if it caused soil toxicity (PRIN PNRR 2022M7S2J SeaForSoil Project). To achieve the aims, sixteen marine biopolymers, such as cyanobacterial (BC1–2) and macroalgal biomasses (BM1–4), biosurfactants (BS1–4) and exopolysaccharides (EPS1–6), were added to soils (1% weight). Based on their soil wettability and phytotoxicity, BC2, BM2, BS4 and EPS6 were selected as the best biopolymers. Then, their toxicity (mortality, biomass, and body length, after 7 and 28 days ) was tested on Steinernema feltiae S. and Eisenia fetida S. Moreover, the effects of the selected biopolymers were also evaluated on the growth of the crop Lactuca sativa L. (height, root and shoot biomasses, and leaf traits in two-month-old specimens). The findings highlighted that the addition of BS4, BC2 and BM2 caused slight toxicity on both S. feltiae and E. fetida. The addition of EPS6 significantly increased the biomass and length of E. fetida and promoted the crop production of L. sativa. In conclusion, considering the different sensitivity of the tested organisms to the addition of the investigated polymers, the exopolysaccharide EPS6 emerged as the most promising biopolymer in improving soil water retention and enhancing crop production without causing soil ecotoxicity.