I'll give you a serious answer since everyone already covered all the NON-serious ones!!
Do you mean NEW manufacture inboard and I/O boats? or what was the biggest OEM I/O inboard engine from the past?
Mercruiser used 454/502 (7.4/8.2L) ....... and the 496 (8.1L) engine for years as did Volvo.(Right now, Mercruiser indicates that an 8.2L MPI engine is current) OMC used the 460 (7.5L) with the Cobra for about 3 years (had one....UGH!)
GM Powertrain stopped producing the 454/502 some years back I believe, in favor of the 496 which may be still produced.
There's literally hundreds of thousands of these engines in wrecking yards and rebuild shops all around the country.
Are you wanting to repower a boat?