Here is the Michigan law. You must have mufflers. I do believe however that Donkey ****s with water injectrion would be legal under the current law. They will most likely not pass the proposed "Pass-by" law if it is implemented. Under the current law even the automatic bypass mufflers are illegal. Also, different than some states, you must have mufflers!
Cut and pasted from MCL law library
6.10 Equipment Requirements and Required Personal
A. Misdemeanor—Equipment Requirements
MCL 324.80156 details the sound requirements for motorboat mufflers and underwater exhaust systems. MCL 324.80156(1) states:
*MCL 324.80156(2) allows the DNR to establish rules for alternative sound level tests and maximum decibel levels.
“Subject to subsection (2),* a person shall not operate a motorboat on the waters of this state unless the motorboat is equipped and maintained with an effective muffler or underwater exhaust system that does not produce sound levels in excess of 90 dB(A) when subjected to a stationary sound level test as prescribed by SAE J2005 or a sound level in excess of 75 dB(A) when subjected to a shoreline sound level measurement procedure as described by SAE J1970. The operator of a motorboat shall present the motorboat for a sound level test as prescribed by SAE J2005 upon the request of a peace officer. If a motorboat is equipped with more than 1 motor or engine, the test shall be performed with all motors or engines operating. To determine whether a person is violating this subsection, a peace officer may measure sound levels pursuant to procedures prescribed in SAE J1970, issued 1991-92.” A person who violates MCL 324.80156(1) is guilty of a misdemeanor punishable by: • imprisonment for not more than 90 days; and • fine of not less than $100.00 or more than $500.00. MCL 324.80156(6). In addition, the person must install a muffler or underwater exhaust system that meets the act’s requirements before putting the vessel back in use. Id.
R 281.1238 Mufflers; cut outs.
The exhaust of all internal combustion engines used on any waters of this state shall be muffled effectively by equipment so constructed and used as to muffle the noise of the exhaust of the engine in a reasonable manner. The use of cut outs, bypasses, or similar devices is prohibited, except for vessels competing in a boat race or regatta sanctioned by the department under sections 151 and 152 of the act. A modern device, underwater exhaust system or a system discharging cold water through the exhaust of an inboard engine, each of which is capable of muffling the noise of the exhaust of the engine in a reasonable manner, shall be considered as meeting the requirements of this rule.