Rich,
Firstly, Welcome!
Coiled on deck sounds like it;s wa-a-a-a-ay too long. As has been said, a short one is your answer. On the plus side, as you are changing due to incorrect length rather than "it's seized" you could punt the old one in the "Bits'n'pieces" section....
Now, to your cable:
There are, as intimated two types - hydraulic & "mechanical" - see above to tell them apart.
Assuming it's mechanical:
The one thing to be wary of, especially with an SR4, is that there was a "low power" system out there, rated to something like 50Hp. Now, SR4s are Max 50Hp, so just be aware that you might have the "low power" helm in your console. I discovered that the hard way after I found my Boat was originally supplied with a 40Hp. I had to up the size of the helm (the gubbins behind the wheel) for the Merc.
Both the cable and the helm should have numbers on - if you take them with you (or have them to hand if online!) you should be OK. - I would google any numbers you find on the helm unit -that should get you enough descriptions and "works with x cable type" kind of links.
To remove it at the ngine end, unbolt at the moving end that pokes out your engine tilt tube , then undo the big nut on the tilt tube at that end. it should then slide out, but may need some gentle persuasion.....
Onece you have the engine end free, undo the nut holding the outer sheath to the helm gubbins, and pull, - you may need to turn the wheel to help it until the entire rack of teeth is out.
The fun bit is figuring out what angle the wheel should be when you start feeding the cable teeth in to make sure it all lines up when "dead ahead"
Tigga
it's a steering cable 
