The search for environment-friendly and non-toxic antifouling (AF) paint components has led to the investigation of natural products from seaweeds. The defence metabolites used by algae to deter unwanted epibiosis have potential for harnessing and use in AF applications. Crude algal extracts may provide a suitable mixture of compounds with AF potency. Crude ethanol extracts of the macroalgae Chondrus crispus (Rhodophyceae), from both dried and fresh sources were tested and compared using bioassays based on five marine bacterial strains, five phytoplankton strains and two macroalgae to assess the AF efficacy. Dried extract from the algae had a lower minimum inhibitory concentration at 25 μg mL−1 against the growth of bacteria and phytoplankton species than that from the fresh source. Macroalgae tests indicated that the extracts had an anti-germination activity 25–50 μg mL−1 against both Undaria pinnatifida and Ulva intestinalis spores. A field trial of AF paint incorporating crude extract indicated an initial AF potency lasting six weeks.