Thanks for that info. I am planning to address the cooler replacement, at least to carry a spare.
I cannot yet say for certain that the cooler is operating at maximum efficiency, however I am sure we don't have a leak of fluid or any seawater ingress. When we reached shore after the event, I drained 2.05 litres of fluid from the transmission. It looked perfect. I have now changed the fluid about 4 times, and have not noticead any signs of seawater.
I'm fairly confident the biggest source of heat in the event was the brake disk (which is now beautifully polished !) which transferred to the transmission, metal to metal. Immediately after the event, the Volvo temp sensor was showing 80deg which is fairly normal for running at 2000.