Like that environmental advice with a skew... Take nothing, have no footprint.
You probably need 2 TShirts, 2 pairs shorts, swimmers, toilet bag the size they give u on a plane.
BUT they may expect u got go out on the town so ask them, and bring a nice shirt, good shorts.
best to bring money and buy stuff u require. For example, the boat may have 1 gallon bottle of sunscreen and feel 3 extra small ones a pain in the bathroom... So find out before, or buy some there.
Same with extra clothes, a Tahitian shirt may end up as a v nice souveneer.
A 40 foot boat is very small and even a TShirt left in a common area is quite distracting.
SAVE WATER!!! Practise at home having a shower using 10 seconds of water to get wet, then scrub, then 15 second rinse. Practise cleaning your teath without any water at all! Tastes great! Yum!
If they troll a line and catch a fish YOU clean the deck. Thats the hardest, mist horrible job and your hosts will appreciate it immensely.
Be the first to pitch in and help, but ask first. Guests on my boat can not do the dishes because of the unique, stupid, tap set up, so till I change it I would hate for a guest to touch it... Lucky guests
Don't tweek the bloody sails! Leave your racing prowess at home unless its truly-rooly appreciated. If you power up a boats sails that is 6,000 nms from a decent chandlery you may break something that can not be replaced until they get to Australia. Its not a saturday afternoon race... Its a huge ocean where long term safety is utterly paramount.
Be a good guest and your hosts will become relaxed around you

and invite you back.
P.S. Even if you are seasick, hungover, bleading from 16 bumps, always have a smile and pitch in for work even whilst chundering and feeling like death warmed up.
Mark