Problem solved: Blocked exhaust elbow.
For future searches on this topic;
According to the diesel mech they do block up every 5-7 years. The boat is located in Sydney, Australia used all year around by the three families sharing it. The blockage happened at around 450 hrs of usage on a 2007 Bavaria 37. This boat has sail drive so in this case there is no traditional prop shaft that could cause that type of issue.
Thanks for all the suggestions regarding this!