In order to get a meaningful answer you need to edit your question with more details. like type boat: sail or fishing. Then the type drive, like fixed, I/O, jet Gas or diesel and location like FL or CA Thanks!.........EDIT........ Thats a little better, This could be OMC or Mercruiser. A marine replacement could run $5000 for the engine and installation...
about half that for a "long block" but that would assume the old engine is in good shape. That would mean a profesional tare-down, and if its not worth repairing thats just wasted money. This is a GM block that was used by both OMC and Mercruiser........I think you can still get long blocks, I don't think new complete engines are available, and I don't know fore sure what new engine will bolt up. best get someone to look your engine over.....I wouldn't be surprised if you could pick up a used boat cheaper than repairing yours, then sell your off for parts.