i agree with the boat mechanic , my little mercruise 151 4 cyl s oil is always really hot even thoght the motor hardly ever runs warm as it has all the cooling of whatever lake i am in. my boat does not have a oil cooler but the theory is correct the only way to cool the oil is through contact with the block or through the pan.
but the idea of a high flow oil pump may also cause cooling problems. i would still think a larger cooler wouldnt hurt the problem and probably solve it.