We took the Voyager 3 on a day trip from St. Maarten to St. Bart's. This boat is a large catamaran designed to handle the open ocean between these islands, and while I do often get seasick and took dramamine before this trip, I actually regretted it later because the ride was fairly smooth and actually pretty fun. (It's been a while since I took dramamine, and I forgot how drowsy it makes you.) You do feel the waves, and the closer to the front you sit the more exaggerated they are, but you're not being tossed all over the place as I actually expected.
The interior is like an airplane cabin, although with a bit more legroom. They do have a drink service if you want to buy one - we didn't in either direction because it's just a short ride. (To be honest, I think a drink service on a 30 minute ferry ride is a little unnecessary, but hey, it's the islands - some people want a beer in their hand at all times). There's also a "business class" cabin upstairs but I can't see paying the extra fee for that for a 30 minute ride.
There are also exterior seats, but these have both their good and bad points, mostly depending on the weather (if it rains, you will get wet; if it's a little chilly, you will get very cold in the strong wind). We sat in a rear-facing exterior seat on the way back to St. Maarten and liked being in the open air on the ocean, but the engines are also very loud back there and the crew has to store large pontoons to protect the hull when it's docked right on the floor in front of the seats, which can be a little annoying.
Be aware that traveling between islands requires all the same immigration rigmarole whether going by ferry or plane, so you need your passport and you need to be prepared to stand in long, slow moving lines at both ends of the trip.
Lastly, I see some bad reviews for service changes and cancellations and a lack of refunds afterwards. We had no problem at all, but this is why you pay by credit card. If they "steal" your money, just dispute the charge and get it back. No one can take money for a service not provided if you pay with a major credit card; that's one of the protections you have as a card user.
If you own or manage Voyager Boat Excursion, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.