90 percent of the 6.5-inch speakers used in a boat have a 6.875" grill O.D., a 5.0" opening (a select few have as large as a 5.625"), and a 6.125" hole pattern diameter. You might see a marginal deviation.
A question was asked above if you could use a light diffuser ring as an adapter. Sure, if it is used solely as a depth reducer. But, as a means to reduce the basket diameter for a smaller hole....absolutely not. That could in some speakers greatly reduce the radiating surface area of an infinite baffle speaker into the gunnel cavity and seriously hurt the midbass output/response.

David