if you have velvet vee drives, it's a known problem. Many of these 98 thru 2000/2001 failed usually with fairly low hours. on my prev. boat, one failed at 100 hours, the other one aroudn 170. first one full warranty, the second one out of warranty but merc provided a new tranny, i paid the labor (about $1500 or so)
The first one jsut started running hot at fast cruise (3600) at low cuise 3200 it was fine... second one first failed with a bang, it was the pressure relief valve in the oil pump. replaced that ($800) but the drive started overheating soon after.
Merc also provided larger coolers...
if you have enough room around the engines and tranny you dont' need to pull the engine, they can just discon the shaft and replace the tranny.
no other symptoms before failure, fluid was fine. the problem is the oil pump and press relief valve.
mos shoudl have failed by now, unless they are low hours and the boats where mostly run at low speed.
good luck.