Volvo Penta and Mercruiser engines are both made by General Motors.  They're the same.  How does a boat owner looking for an engine not know this?
Also there is no such thing as a "totaled" engine.  Usually a rod breaks or bearing spins, but everything else is fine.  So you buy something called a "short block" or "rotating assembly" and just replace that.  
Double check but I'm 99.9999999% sure this year automotive 305 is what VP/MC used.  Car/boat same engine.  $625
http://www.ebay.com/itm/Remanufactured- ... ff&vxp=mtrTo pay an honest shadetree mechanic to switch it all out you're probably looking at another $1000 or so for the labor.  I rebuilt my first 305 when I was 15 with my father, it's monkey work.  With a forklift and hand tools a mechanic could start the project on a Friday and have you on the water for Sunday Funday.