I still think you might want to check the damper plate. I suspect the sound you are hearing when you engage the transmission and the backfiring are separate issues.
Hopefully it's not too expensive or too long before you get the boat back.
1990 Prostar 190, 351 Indmar SO, bought as an empty shell and rebuilt over the winter (04/05). Sold in May 2009
New boat - 1993 Prostar 205. Bought in better shape than the 190 was but still needed some work.