What is the difference between a boat inboard motor and a car motor?

I need to replace the motor in my boat and I have a Ford 300 6 cyl. Engine in my back yard. Can i use it for the boat? What needs to be modified?

No body mentioned the marine head gasket yet. Otherwise you get about a year of service in salt water and it fails. Also some engines had a steel backed seal in the water pump which went away even faster. Good idea to get a marine/ RV cam for the engine to recover some low end torque. That 300 was a heavy S.O.B. So no worries about running a raw water coolant system. The exhaust manifold price will make you gasp. You might change out the fuel pump for a marine style which funnels any fuel from a broken diaphramn into the carb instead of the bilge. While the head is off -good time to check the valves, and look down to the timing chain set and see if it is nylon coated or a real chain on double rollers.
The carb is another issue. The marine carb floods back into the carb where the automotive pours fuel on to the manifold.
Last but not least is the vent on the starter must have some sort of fire screen in it.
Hope this isn't discouraging. Best bet is to find a rust bucket in a marine grave yard and scaveneng the bits.***:D

Bettye, don't be discouraged by those who are obviously NOT well rounded as sailors…

This is what you are going to basically do: get a keel Cooler (radiator) to install on the outside of your hull and piping to modify your exhaust so it discharges from a stack (muffler) away and above personnel onboard. Use a website called "Boats and Harbors" to purchase your keel cooler and insure your piping for stack (muffler) is insulated so nobody gets hurt. Get a gas detector and fans for your engine room to expel hot air. Contact USCG safety office and they'll give you all the safety info for setting up engine. Moreover, you'll be surprised at how many boats, especially fishing boats, are in USCG registry, set up with your idea… Sad that all sailors are not as well rounded as they should be and give ignorant advice about set ups they no NOTHING about! Please accept my sincere apologies for the fact ANYBODY is allowed to post here. Notwithstanding, your engine will work for "years on end"… Note that saltwater NEVER touches the engine internally!

Annmarie answered most of it, But there has never been a marine Ford 300 Therefore you will have lots of problems finding correct parts. You need to tell us the motor that you are replacing, as there are all kinds of places to get remanufactured marine engine blocks.

My current (old) inboard motor on my sailboat is based on an engine as used in a Ford Cortina, converted by a now-defunct UK firm from a version Ford sold for industrial use. As I recall, the differences are:
- marine inlet manifold, with carb and air filter set to be horizontal with the engine installed normally (slanted down at the stern)
- marine water-cooled exhaust manifold
- marine mechanical-drive lift fuel punp, I think
- marine direct-drive water pump (Jabsco) (raw water cooling on that model)
- different cooling water routing and thermostat
- (now) spark-proof alternator
- sump oil drain routed via an easy- access pipe
- different electric harness
I may have missed some; I have a users's manual onboard that lists all the custom parts.