When a 6-pack, second issue, buddy of mine was taking the test for the sailing endorsement, I took the practice test cold, just to see how I would do. I passed. But I have been sailing boats since the 1970's & I've driven well over 50 different boats across a few thousand miles of geographical area, so my on the job experience is fairly good. If you are starting from zero, you will need to work hard to pack enough into your head inside a week to have a good chance of passing. That being said, I know of courses in the Fort Lauderdale area that prep guys for the 6-pack test in a week & most of them pass.
Experience is as important as book learning, which is why the days at sea requirement exists. You need to pass the written test & have a certain number of days at sea before your credential will be issued.
There are different licenses for different size boats, different areas & different specialties. An inland license is only good for inland waterways. A Great Lakes ticket covers inland as well as the Great Lakes. An off shore ticket covers all three. I may not have those names quite right, but the geographic limitations should be pretty close to right.
The lowest credential (other than a launch operator) is a "6-pack" license. This is not an official name, it's just what guys call it. This is what most people start with because you don't need a lot of days at sea to get it & your time on any old little boat counts. A 6-pack lets you take out an uninspected vessel with up to 6 paying passengers. Then there is a 50 ton masters, 100 ton masters, 200 ton masters, unlimited, etc. You can also get endorsements for sailboats, towing, etc on different licenses. There are a lot of guys with 6-pack or 50-ton masters licenses working as lowly crew on 100-ton & 200-ton boats, to build there time on boats of that size to qualify for days at sea for a license of that category. If you want a 200-ton license, you need time on 200-ton boats. The guys that drive cruise ships have unlimited ocean crossing tickets. Those generally take a long time to get.