The transmission oil cooler failed and I have a tranny that has to be replaced.

I had come back from a 2 hr cruise and checked the oil levels on the transmissions. The port side required 1/2 quart while the starboard read empty. I checked again and did not get a read so I tried adding fluid. As I was adding, the fluid was finding its way to the bilge and I knew something unfortunate was going on. My mechanic came down and determined that the fluid was not leaking out but rather the tranny was filled with water and the fluid had no where to go. Per the area marine transmission specialist, I pumped out the water, filled to top with fluid and flushed 2x going into forward and reverse. It seemed to be working and thought that I dodged a bullet.
I took her out for a run in order to change the fluid yet again but after about 20 minutes, it quit.
How does one avoid this mess in the future? Do any of you folks preemptively change the oil coolers for both transmissions and engines? I owned my last boat for 19 years, never changed the coolers and never had an issue.
All I know is that the lousy $140.00 oil cooler is gonna cost me about 5K!
I would be interested in knowing how you guys approach this.
Thanks.





