Water in transmission

Depending on how much water is in the tranny you may be able to run a tube to the bottom of the sump and pump it out also. Don't forget to empty the existing oil filter and then replace with new when you've completed flushing.

I had to flush three times (+ bring it up to temp in between) to get rid of all the emulsified water in the transmission. No long-term damage as far as I could tell.

Keep a watch on it, and even if it looks clean after a few runs, I would change it at least one more time with the filter

How did water get in there in the first place ?

. If you have twins you may consider replacing both coolers if that was your problem.

On a previous boat had high sodium on a gear oil analysis, the cooler failed the pressure test. Mechanic replaced the cooler and changed oil/ filter, said to retest at 50 hrs. Gear was ok, caught the cooler leak in time.

Ahh ...as usual learned something new again on YF...I did know there was a trans cooler somewhere on my DDs , but forgot about them and their purpose and the other reason to check the oil other than just for the proper oil level.
My trans dip sticks are right in my face easy as can be to check , now I'll put them on an "always" basis.

Check the same time you check your oil- that should be every time you start them up even at the dock. You'll never know your oil bled out thru a hose or metal failure unless you look.

Most manufactures also recommend a filter and fluid change every 200-250 hour or 2 years, I believe. My ZF's do. Mine use straight 30w

Or, if the gear lost oil, you'll get out of the slip and have no forward or reverse on one engine and a major OH NO moment.