As a comparison to what they are, you could look on the Microsoft Flight Smulator website for the SDK on there, you'll see that is a big collection of software tools and instruction guides for how to mod stuff and add it to the sim, so there are things like a scenery SDK, a missions SDK, an add-on aeroplane SDK etc, etc. They can be something as simple as a plug in for a 3D application that lets you export a 3D model in the right format for the sim if you fancy knocking up a plane for yourself, to a complex set of recommended guidelines on an html document or pdf for professional add-on development companies, to ensure that their add-on will be compatible with the main sim.
There's usually two schools of thought from the developer of a game or sim: A) let's release an SDK and make our game hugely popular with users at large so they can add masses of content (this has been a tradition with MS flight simulator for years, and an SDK is pretty much expected with every release of it, the SDK often gets released to the public slightly later than it is made available to professional add-on companies), and B) let's keep it all under wraps and control how the sim develops.
The A approach tends to ensure a big community and longevity, but it also means that there is often very little control over the quality of many add-ons. The B approach tends to ensure that things which get added, either by the original developers (i.e like the upcoming SH4 add-on), or by modders who have a crack at doing it despite not having any official help (i.e like GWX and Trigger Maru), are usually of a generally higher standard, as they are either done properly with all the tools that built the original sim, or they are doggedly pursued by people who are prepared to probe how the sim is built and use their (often considerable) knowledge to add stuff by 'reverse engineering' the sim.
Some sims are famous for being tough to crack and efforts for ones like this are often strongly disapproved of by the devs, an example of this being the combat flight simulator IL-2, anything other than repaints or missions designed with the in-game mission creator are strongly discouraged. The Silent Hunter series developers tend to be a bit more pragmatic, in that, while they do not make life easy for modders by releasing tools to assist them, modding is tolerated despite the fact that technically, it is a contravention of the EULA agreement which is part of the software license you agree to when you install the sim. An attitude such as this to mods is usually conducive to the sim having some longevity too, despite such mods quasi-illegal status.
So in short, modders would like an SDK, but they are unlikely to get one for SH.
Chock