Well, one was WernerSobe's
Natural Sinking Mechanics (NSM), with the download link
Natural Sinking Mechanics 4.0, which should be the most recent version. Be sure and read the ReadMe (Rockin Robbins) with the download ~and~ the ones with the mod. That version is for v1.5 of the game, which has the Uboats. If you have an earlier version of SH4, you might have to do some searching for it. I could not find any others on SubSim, but my "Search" efforts are 'infamous' for how ~bad~ they are...
- but you could try the
WernerSobe folder at s7rikeback's Silent Hunter Mod Collection, and download the v2.6 to give that a whirl. Again, the documentation stuff should state what it can run in. One important thing to remember about the mod is that it changes files that a lot of other mods change. If you apply this mod, it has a given Roster. Anything not in the mod's Roster does not get changed. Several large mods incorporate a compatible version of this in their files, so if you are doing other mods, be sure and read their documentation...
PS: The easy way to find your SH version is to use Windows File Manager to go to the Silent Hunter folder, right-click on the SH4.exe, then left-click on Properties, then click on Details, and about in the middle of that dialog should be the version number.