On-duty marine and helicopter officers boarded the boat owned by a friend of state Delegate Jon Cardon Aug. 7, with one report indicating officers pretended to search the vessel when Cardin proposed, The Baltimore Sun reported Tuesday.
A police spokesman said the department was investigating the possible misuse of police resources.
"Definitely there was some poor judgment exercised by some officers," Anthony Guglielmi said. "The police department is not in the business of renting out the helicopter and the boats for bachelor parties and birthdays. We're in the business of upholding public safety in Baltimore."
State Delegate Curtis S. Anderson said he was shocked by the apparent indiscretion and angry that officers were diverted from their primary mission of protecting citizens against crime.
Cardin, an attorney who represents Northwest Baltimore and nephew of U.S. Sen. Ben L. Cardin, D-Md., did not return calls Monday.
In a statement released last week, Cardin described the actions by police as a "5-minute safety check" and didn't say whether helicopter police participated, the Sun reported.
"During the evening, I was focused on making my fiancee's night perfect," his statement said, adding she accepted his proposal. "In retrospect, I should have considered that city resources would be involved and used better judgment to put a stop to it."
Cardin said he would reimburse the city "for whatever costs they deem appropriate."