ok , im an idiot.
can someone tell me what this is.
radiator on a boat?

Close enough.

What you have is a water to water heat exchanger. You run coolant in the engine and raw water to cool that.

Oversimplified but that's about it.

Try this website
http://i-netmarine.com/fwc/MerCruise...half%20systems
Bryan

thanks fro the info guys , one other question , is it a plus or minus to the value of a boat?

It depends on where you boat.
If it's salt water it's a plus, you don't have to worry about salt clogging the engine, internal corrosion and there's less to flush out. If the system fresh water cools the exhaust manifolds too (some do, some don't), it's a real big plus. The manifolds last a lot longer that way.
With a closed system you don't have to worry about draing the block prior to winter.

I'm sure there's alot more advantages to it, just can't think of them.

thanks iggy ,
i only boat in fresh water but the boat im looking at buying is a salty.

im of the thinking that less bells and whistles the better.
less to repair however not having to draining the block sounds great.