I roughly priced what it would take to put in an LS vs rebuild the SBC in mine.

Just about all the marinization companies (PCM, Crusader, Marine Power) offered the 6.0L in the mid to late 2000's so the parts are out there to DIY an LS swap. One stop shop would be Hardin Marine, they have just about everything available but on the pricey side.

Piece meal the parts and I came to at least $2500
Manifolds $800
Risers $400
O2 oxygen plate $180
Flywheel $80 (2001 4.8L Chevy 1500)
Motor Mount Adapter $50
Lift Pump $54.99
High Pressure Fuel Pump $100
Fittings $300
ECM Tuner $400
ECM Credits $100

I know there are other things that I have missed like seawater pump mounting, hoses and unforeseen expenses.