A method for assessing the severity of an underwater dive, the method comprises the calculation of a dive severity index, which calculation is performed by carrying out the following steps: a) determining the gas breathed by the diver, b) measuring the dive time, c) determining the depth profile of the dive, d) calculating a dive severity index based on a function of a), b) and c) e) assessing the severity based on the result of the calculation of the dive severity index d) the steps a), b), c) and d) being carried out in real-time mode, at each instant of the dive time of the diver, and the calculation d) being performed according to the following formula: DSI = k 1 ∗ f GAS ∗ f D ∗ f T where: DSI is the dive severity index, k1 is an arbitrary constant, f is a function of the inhaled gas, f is a function of the dive depth, f is a function of the dive time and wherein there is provided a step changing the parameters of a decompression algorithm according to the value of the dive severity index. and wherein step d) relating to the calculation of the dive severity index is carried out according to the following formula: DSI = 5 ∗ FN 2 / 0.79 D avg 10 + 1 ∗ T where: D avg is the average dive depth, in meters, updated at each instant T is the dive time in minutes FN2 is the nitrogen fraction in the gas breathed by the diver. a) determining the gas breathed by the diver, b) measuring the dive time, c) determining the depth profile of the dive, d) calculating a dive severity index based on a function of a), b) and c) e) assessing the severity based on the result of the calculation of the dive severity index d) the steps a), b), c) and d) being carried out in real-time mode, at each instant of the dive time of the diver, and the calculation d) being performed according to the following formula: DSI = k 1 ∗ f GAS ∗ f D ∗ f T where: DSI is the dive severity index, k1 is an arbitrary constant, f is a function of the inhaled gas, f is a function of the dive depth, f is a function of the dive time and wherein there is provided a step changing the parameters of a decompression algorithm according to the value of the dive severity index. and wherein step d) relating to the calculation of the dive severity index is carried out according to the following formula: DSI = 5 ∗ FN 2 / 0.79 D avg 10 + 1 ∗ T where: D avg is the average dive depth, in meters, updated at each instant T is the dive time in minutes FN2 is the nitrogen fraction in the gas breathed by the diver. DSI is the dive severity index, k1 is an arbitrary constant, f is a function of the inhaled gas, f is a function of the dive depth, f is a function of the dive time and wherein there is provided a step changing the parameters of a decompression algorithm according to the value of the dive severity index. and wherein step d) relating to the calculation of the dive severity index is carried out according to the following formula: DSI = 5 ∗ FN 2 / 0.79 D avg 10 + 1 ∗ T where: D avg is the average dive depth, in meters, updated at each instant T is the dive time in minutes FN2 is the nitrogen fraction in the gas breathed by the diver. D avg is the average dive depth, in meters, updated at each instant T is the dive time in minutes FN2 is the nitrogen fraction in the gas breathed by the diver.