What you have (by the sound of it) is a hydraulic pump on the end of your engine, driving a hydraulic motor(s) that spins the propellor(s). In that installation, if the oil doesn't get so hot it bakes the seals etc you do not need a cooler. You will not suffer any power loss just because the oil gets a bit hot.
"Hot" in this context depends on manufacturers spec, but if it's less than say 110 degrees Celsius dont worry. I'd be amazed if it gets anywhere near that, I mean if it's that hot you would not be able to put your hand on the piping.