Offshoreonly.com - Bravo 2 noisy when hot

We own a 24' fishingboat which is powerd by a Yanmar 200Hp engine in combination with a Bravo 2 X drive.
This setup is almost 4 years old and had run about 100 hours.

The strange thing I noticed is when we go on the water for a small trip, everything is ok.
But when we go out for a longer trip let's say 1.5 hours at 80% of throttle, the drive is making noise when we climb into the waves. The is noise is like the noise from a worn gearbox, this noise is also hearable when we are in gear at idlespeed.I know when we let it run on the shore bravo's are always making noise when in gear, but not that loud.

I changed the oil three times now, but it comes out as brand new. I did a quick view under the top cover and everyting seems almost new.

The only thing I can think off is the oil is getting thinner when the drive gets hotter.
Because the boat is the whole year in the water(salt) we have to use Anti-Fouling paint against sea-growt. But at the top of the drive this paint is cracking due to the heat.
So I am thinking abut buying/fabricating a driveshower or is this a bit overdone with 200hp?, but it is a diesel with only 3200rpm and a fair bit of torque.

The most stupid thing is, that I am a Merc mechanic but I don't have that much experience with bravo drives. There are only a few bravo 2's in the region. And if one is broke I can rebuild to factory spec's, no problem. But we don't have any try and error experience, just ordering the origal parts and fixing the drive.
But I don't want it to fail because the waves at sea are very short over here at 50 miles from the coast. And it isn't a pleassure to be over there with a bout that can't manouvre.

If I give a call to Merc Europe they simpely can't help me. I think they don't have any technical knowhow about these drives, they only thing they can do is paperwork and even then they screw up.

Thanks,