An underwater hydraulic power unit leak detection apparatus, comprising: a) a pump and a prime mover operating the pump, the pump supplying hydraulic power to a hydraulic device through a first and a second hydraulic fluid flow line (31, 32); b) the hydraulic device comprising a hydraulic cylinder having a pushrod that moves between extended and retracted positions, the pushrod having a substantially uniform cross sectional area; c) the hydraulic cylinder comprising a cylinder having an interior that includes a first or pushrod retraction chamber and a second or pushrod extension chamber on opposed sides of a piston, the first or pushrod retraction chamber being on the side of the piston that has the pushrod, the second or pushrod extension chamber being of a large cross-section; d) the first or pushrod retraction chamber being receptive of hydraulic fluid when retracting the pushrod and the second or pushrod extension chamber being receptive of hydraulic fluid when extending the pushrod; e) a hose reel including a first hydraulic hose connected to the first or pushrod retraction chamber and a second hydraulic hose connected to the second or pushrod extension chamber; f) the first hydraulic fluid flow line being connected to the hose reel and the first hydraulic hose that supplies hydraulic fluid to the first or pushrod retraction chamber, the first hydraulic flow line comprising a first flow meter; g) the second hydraulic flow line being connected to the hose reel and the second hydraulic hose that supplies hydraulic fluid to the second or pushrod extension chamber, the second hydraulic flow line comprising a second flow meter; h) a computer that continuously monitors the first and the second flow meters (45, 46) in order to calculate a ratio between monitored fluid flows in the first and the second hydraulic fluid flow lines (31, 32); i) the piston having a piston diameter (Dp) and the pushrod having a pushrod diameter (Dr) used to calculate an ideal, non-leaking ratio between a flow rate of a volume of fluid flowing into the first or pushrod retraction chamber of the hydraulic cylinder and a flow rate of a volume of fluid correspondingly flowing out of the second or pushrod extension chamber of the hydraulic cylinder or between a flow rate of a volume of fluid flowing out of the first or pushrod retraction chamber of the hydraulic cylinder and a flow rate of a volume of fluid correspondingly flowing into the second or pushrod extension chamber of the hydraulic cylinder; and j) the computer being operatively connected to the prime mover so that the computer can deactivate the prime mover when the ratio calculated in step "h" varies from the ideal, non-leaking ratio calculated in step "I" by more than a predetermined amount. a) a pump and a prime mover operating the pump, the pump supplying hydraulic power to a hydraulic device through a first and a second hydraulic fluid flow line (31, 32); b) the hydraulic device comprising a hydraulic cylinder having a pushrod that moves between extended and retracted positions, the pushrod having a substantially uniform cross sectional area; c) the hydraulic cylinder comprising a cylinder having an interior that includes a first or pushrod retraction chamber and a second or pushrod extension chamber on opposed sides of a piston, the first or pushrod retraction chamber being on the side of the piston that has the pushrod, the second or pushrod extension chamber being of a large cross-section; d) the first or pushrod retraction chamber being receptive of hydraulic fluid when retracting the pushrod and the second or pushrod extension chamber being receptive of hydraulic fluid when extending the pushrod; e) a hose reel including a first hydraulic hose connected to the first or pushrod retraction chamber and a second hydraulic hose connected to the second or pushrod extension chamber; f) the first hydraulic fluid flow line being connected to the hose reel and the first hydraulic hose that supplies hydraulic fluid to the first or pushrod retraction chamber, the first hydraulic flow line comprising a first flow meter; g) the second hydraulic flow line being connected to the hose reel and the second hydraulic hose that supplies hydraulic fluid to the second or pushrod extension chamber, the second hydraulic flow line comprising a second flow meter; h) a computer that continuously monitors the first and the second flow meters (45, 46) in order to calculate a ratio between monitored fluid flows in the first and the second hydraulic fluid flow lines (31, 32); i) the piston having a piston diameter (Dp) and the pushrod having a pushrod diameter (Dr) used to calculate an ideal, non-leaking ratio between a flow rate of a volume of fluid flowing into the first or pushrod retraction chamber of the hydraulic cylinder and a flow rate of a volume of fluid correspondingly flowing out of the second or pushrod extension chamber of the hydraulic cylinder or between a flow rate of a volume of fluid flowing out of the first or pushrod retraction chamber of the hydraulic cylinder and a flow rate of a volume of fluid correspondingly flowing into the second or pushrod extension chamber of the hydraulic cylinder; and j) the computer being operatively connected to the prime mover so that the computer can deactivate the prime mover when the ratio calculated in step "h" varies from the ideal, non-leaking ratio calculated in step "I" by more than a predetermined amount.