Well you didn't say what year/model it is.
The riser is what usually keeps the water from the exhaust system out of the exhaust.
If it's a Cobra there's probably a shutter or "flapper" in there that helps.
Also, if you have a cracked head/block it's probably not worth fixing at all unless you can find a junk yard engine.
If it's a head gasket you might be able to make it all work. I don't know if you can buy a manifold for it.
The following picture is from Doug Russell marine for an 88 3.0L Cobra.
http://www.dougrussell.com/partscatalog ... 88,489,490#48 is the riser and #81 is the "shutter" ....OMC calls it an "exhaust seal"
If you have to buy a manifold and riser you'll be into it for about $700 if you get it from Doug Russell (if they have it)..... There's a lot of sources out there for this stuff. OMC has been out of business for more than 10 years.
The 3.0L engine is a very good engine and is still well supported by Mercury and Volvo.
A compression check should rule out a cracked block/head or failed head gasket. It won't tell you anything about a failed manifold or riser.
If the riser gasket (#9) fails for example it can allow water to drain back into an open exhaust valve and into a cyl......that will cause a hydro-lock when you try to start it later.
