I agree with the above. Just a bit more info. A "sea service day" is defined as a minimum of four hours in which the boat leaves the dock. An example could be as follows, One hour at the dock to prepare to sail, two hours of sailing away from dock, and one hour to putting the boat to bed at the dock. Those short evening sails can sure add up fast.
I have a 500 ton near coastal license. I went to a school for my 100 ton. Then after 1800 days of captaining a 100 ton boat I qualified to upgrade to the 500 ton. I did not go to school for this license, but I did order a home study guide. I found this most useful and very thorough. It was pricey, about $800 for all materials, but considering it is my livelihood it was well worth it. The company is Houstin Marine. They are easy to find on the web and have lots of info on what you may need. I recommend them.