The 'U-Boat add-on' is Patch 1.5. It upgrades several items in the game. The u-boat war is just type IXs in the Pacific, but the Operation Monsun supermod makes a complete u-boat experience similar to SH3.
You can download the U-Boat add-on from Amazon for $9.99, or you can download the entire Gold Edition for $19.99, or you can buy it used on disc for $17.99. Whichever you choose, if you go to Amazon through the 'Quick Links' link at the upper right side of this page Subsim gets a small cut, at no extra cost to you.
http://www.amazon.com/s/ref=nb_sb_sa...t%20hunter%204