We just did this last month (May 2021) and loved it! A few things
We chartered through iYachtClub - most (all?) of the catamarans are chartered by multiple companies. I think iYachtClub brokers directly with the owners so the rate is a little less than chartering the exact same boat through another company? We also liked that they tried to match us with a crew that would be a good fit. I think most of those companies are sensitive to the age of the boat - ours didn't offer anything older than 5 years and ours was a matronly 2017 model (but still very nice).
Our boat was 50' (3 cabins) and the water was smooth, but you if you have any sensitivity to seasickness you will feel it, in my opinion. Take the Dramamine, use the patch or try the bracelet with the little bead. It was not terrible for me, but I did feel like I was moving for several days after we got off the boat. I'm sure the coctkails were a contributing factor as well.
As a poster above said, pack EXTREMELY light. I consider myself a very light packer - I can go to Europe for two weeks and not check a bag - but this trip calls for a whole different level of light packing. I took a different swimsuit for each day (7) plus clothes to wear each way on the plane and one more super casual outfit and I probably could have cut that in half.
The food was fabulous - our chef/first mate was very talented and every meal was gourmet. Delicious! She is a bartender in Martha's Vineyard in the off season and kept a drink in our hand at all times. Our boat had Yetis for us to use - you might ask your crew if they have them so you don't bring one with you.
I did not know until we got on the boat that you can't put toilet paper in the potty, which was pretty gross to me. It gets fairly warm below deck during the day when the captain powers the generator down. Warm bathroom, used tp barf. I would take wipes to use next time. Another husband on our trip said we need to invent something akin to the Diaper Genie and call it the Boat Genie.
Take advantage of the fishing, it is awesome - we caught fish every night and always had a line out. Big tarpon schooling under our boat every night. Our captain's previous gig was as a deep sea fishing guide, so he was way into the fishing, which was fun.
We also learned that sometimes guests bring sussies for their crew. The group previous brought them custom etched Swig cups. We ended up sending them a couple of small things after.
Last thing - we stayed at the recommended "resort" near the airport the first night and wish we had just gone straight to the boat. If you do stay on land, take a cab to Oceana for dinner. It's in the old Russian embassy (I think) in or near Frenchtown and was fantastic and right on the water. We spent one more night on St. Thomas at the end if our trip - but this time at a fancy resort on the other end of the island. Was very nice to have a huge bathroom and big shower transition back to reality. Would do that part again for sure.
We will definitely do a cat trip again - my husband has pretty much been researching our next trip since we got home. And he has decided we will retire on a cat, too
Pic is of our boat (Sea Dragon) taken from my husband's drone.