The reason for the delay isn't really the fault of the DNR. The MN Legislature wrote the law and screwed up the language and then dumped the enforcement in the lap of the DNR. The law was rewritten and passed the House and Senate on May 1, 2018, effective immediately. The new language requires conformance to current UL Standards for use on recreational boats (the same standard that carbon monoxide detector manufacturers use to certify their devices).
The language require them to meet not just UL standards but UL Marine standards. You can't use a detector that is designed for home use it has to be marine rated.
Per MN DNR site: Marine CO detectors must meet ABYC standard A-24 , July 2015 which specifies that detectors must be listed for marine use, including certification by an independent party.
The DNR & Leg. messed up last year when the law went in force and then it was pulled back because of multiple issues. One of which, the law states that Gasoline powered boats need 3 stickers and the DNR didn't even have the stickers before the law was supposed to go in force last year.
Also as Greg said just having them is not good enough, they have to be current and not expired.