A method for testing a filter of an underwater filtration system including at least said filter placed in an aquatic environment, including the steps consisting in: - placing underwater at a depth of at least 500 meters an underwater testing device including: o said filter of the underwater filtration system, o a water inlet which is suitable for introducing into the device a flow of water sampled from the environment of the device o a pipe connecting said water inlet and said filter ; o a discharge pipe downstream of said filter ; o at least one pump which is suitable for filtering of the flow of water across said filter ; and o a means for supplying the device with power; - introducing a flow of water sampled from the environment of the underwater testing device into the underwater testing device via the water inlet ; - bringing said flow in the underwater testing device up to said filter ; - obtaining a filtrate downstream of said filter and discharging said filtrate into the environment via the discharge pipe ; - following in real conditions the evolution of the fouling of said filter , wherein the flow rate of the filtrate is between 0.02 and 5.00 m 3 /h, characterized in that it further includes a step consisting in: - measuring the difference in pressure between the upstream side and the downstream side of said filter(s) ;and - recording said measurement of the difference in pressure in a storage means situated in the underwater testing device or transmitting said measurement of the difference in pressure to a receiving means situated outside the underwater testing device , a step consisting in injecting a biocide into a pipe which connects the water inlet and the filter , the filter is a nanofiltration membrane. - placing underwater at a depth of at least 500 meters an underwater testing device including: o said filter of the underwater filtration system, o a water inlet which is suitable for introducing into the device a flow of water sampled from the environment of the device o a pipe connecting said water inlet and said filter ; o a discharge pipe downstream of said filter ; o at least one pump which is suitable for filtering of the flow of water across said filter ; and o a means for supplying the device with power; o said filter of the underwater filtration system, o a water inlet which is suitable for introducing into the device a flow of water sampled from the environment of the device o a pipe connecting said water inlet and said filter ; o a discharge pipe downstream of said filter ; o at least one pump which is suitable for filtering of the flow of water across said filter ; and o a means for supplying the device with power; - introducing a flow of water sampled from the environment of the underwater testing device into the underwater testing device via the water inlet ; - bringing said flow in the underwater testing device up to said filter ; - obtaining a filtrate downstream of said filter and discharging said filtrate into the environment via the discharge pipe ; - following in real conditions the evolution of the fouling of said filter , wherein the flow rate of the filtrate is between 0.02 and 5.00 m 3 /h, characterized in that it further includes a step consisting in: - measuring the difference in pressure between the upstream side and the downstream side of said filter(s) ;and - recording said measurement of the difference in pressure in a storage means situated in the underwater testing device or transmitting said measurement of the difference in pressure to a receiving means situated outside the underwater testing device , a step consisting in injecting a biocide into a pipe which connects the water inlet and the filter , the filter is a nanofiltration membrane. <EOS>