DH and I regularly go on two week narrowboat trips. I used to find that I took too many clothes. Now, I know I need two complete changes of clothes (including shoes, waterproofs, hats and gloves). I fell in the canal once but, the chances are that you will still get very wet at some time, unless you plan to sit inside the boat and let the others do the outside work. Layers that work together are essential, a tee shirt that works as a vest, a hoodie that you can put up under an outer shell, waterproof overtrousers with leggings that can go underneath jeans. Working locks is hard on clothes, grease stains and snags are common. You get hot working the locks and cold steering the boat.
I mentioned gloves, I always wear fingerless gloves with leather/grippy palms, they make rope handling and using a windlass much more comfortable. I also take slip on shoes for inside the boat which will get very gritty from the tow paths with 4 of you going in and out.
Your boat may be better equipped than ours but we have a problem drying towels.
As to underwear, well, you could take some string and pegs with you and a small bottle of Stergene and rinse out your knicks. However, if you are going with friends, you might find it difficult to find somewhere to let them drip discretely. I take panty liners too but enough underwear to last me the two weeks. The trouble with disposable pants is that you have to dispose of them! One of the biggest problems is disposing of your rubbish. The Canal and River Trust seem to have removed many of the refuse disposal points around the network, meaning you have to carry all your waste with you for several days at a time. We take plenty of plastic carrier bags with us to put our rubbish in and look out for proper refuse points on our travels.
Other things to consider are, for 4 people, you will need to top up the water tank more often and it is unlikely that you will be able to do a two week journey without getting the toilet/s pumped out halfway through.
However, as you've done one week trips, and are going for two weeks, you must already have had some experience which you enjoyed. DH and I really look forward to our times on our shared boat. We are on for another week very soon. Just take it easy. It's all about the journey, not the destination and there will be 4 of you to share the work and fun.