Hi. Welcome to the site. Like Scook said, if it's an I/O check the rubber bellows where the outdrive mounts to the engine. Additionally check any and all holes in the hull (Raw water washdown, cooling water for the engine, hoses, etc. ANY place that an old plastic fitting might have cracked and started letting water in) It's always a good idea to replace all plastic hose fittings with metal ones. Brass, Bronze or Stainless Steel only. 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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