How long before Marine Diesel engine Suppliers are audited?

There might be some confusion of terms, IMO Tier III is not the same as EPA Tier 3.

At the moment there are few, if any marine diesels available that can meet EPA Tier 4 (the closest to IMO Tier III) without exhaust aftertreatment since Tier 4 and Tier III (MARPOL Annex 6) applies to NOx emissions.

As an additional slant on the subject, found a boat that uses the engines in question.

Wider 32

The 32’s standard engine package is a pair of Mercury diesels, and not just any diesels but the new Volkswagen-based V6 TDIs—lightweight, efficient, silky-smooth, and better in all departments than their comparatively old-tech QSD predecessors.​

Think again ... and read the fine print. http://www3.epa.gov/otaq/marine.htm

Haven't you ever wondered why there are so few normally aspirated diesels available these days? The EPA has been "bothering" for a long time.

Utilization of biodiesel and timing adjustments decrease NOx emissions over dino diesel; but that does not address the issue of testing and certification of engines. Urea injection decreases NOx - VW did not buy into the Mercedes-Benz system (which BMW did) and so they decided to comply with the more strict emissions regulations differently. Their efforts failed. The DEF-blue/Urea system of MB is commonplace now; combine this with more strict particulate emissions regulations (since 2008) and we saw a dip in the diesel technical evolution of rising HP/fuel efficiency. Mercedes on road diesels in the years of 05/06 are considered more desirable than the /08+ years until recently partly because of the lower MPG and lack of power of the newer years. The MB OM648 straight 6 diesel engine sold in the "E" class of 2005/06 in the USA was rated at 38/28 MPG with 200 HP and 369 torque. No particulate filter nor Urea injection. It took until the model year 2014 to come close to that same performance while still complying with the new emissions regulations. Used diesel engine powered cars and trucks valuations are noted to support these higher prices- 04-07 prices compared to 08+ show a preference with buyers for the older non emissions controlled units.