I would check the exhaust hoses where they connect to the thru-hull exhaust flanges. Mine only had single hose clamps on that joint that had worked loose over the years I suspect. The flanges on mine had developed a flat spot on the circular hose insert that didn't help either. You may be able to see if that condition exists by looking in from the outside exhaust flap. You'll have to have it in the water and be able to see the connection from the inside (gas tank removed?) to determine if it's leaking or not.
Also, check the speedo tubing and brass tubes for leaks.
1988 Tristar 190 Sport, 351W w/GT-40's, 1:1 Velvetdrive