I have had some experience with this, from small 18 foot boats up to 44 foot boats. The bigger the boat is, the more money that gets involved. On larger boats You positively must figure on replacing everything, on a smaller, simpler boat that you can start on immediately upon removal from the drink, you may be able to save some things. Saltwater is the enemy of electrical devises and especially switch contacts. Most electrical items have springs of some sort in them and even if the thing does work now, springs are not usually made of corrosion resistant metal and they will fail eventually, taking out the devise. Always buy a sinker as a hull and structure only, and it has to be CHEAP. If you include your time for removing all the wiring and components and then replacing them with new or rebuilt, you probably would be money ahead buying a new hull and structure and fitting it out. IMO, anyone selling a sinker without full disclosure to the buyer should be flogged with 6 feet of corroded battery cable. Saltwater doesn't seem to cause a problem with cores or wooden structure, in fact, it will arrest dry rot in it's tracks, so rot after immersion is not a major concern. Cores don't neccessarily become weak from water saturation, it's the attending rot that destroys it. The plastic and foam cores are not affected by rot at all but can still become waterlogged. If the engines can be completely filled and flushed with fresh water, including and especially the cylinders, BEFOR the dry out from the immersion, they won't suffer any long term damage. After a few hours of fresh water rinsing they should be drained and filled to the top (including cylinders) with diesel. The remaining water will settle to the bottom. What you are trying to prevent is a rust ring on the cylinder wall, while filling with diesel turn the engine by hand a few revolutions. You should remove all starters as soon as possible and drop them in a bucket of fresh water. Next day, put them in a bucket of diesel, next day take them to a repair facility and trade them as cores for rebuilt units. Do the same with the alternators. The engines are the easy part.