BTW: I used my AT&T cellphone from the boat in St Croix. You're right there the whole time.
The stability issue on Nekton is always discussed as black and white, when it isn't. You'd have to break it down to "at anchor" and "underway".
I've been on Nekton for St Croix trip. I have been on other liveaboards. My experience was that at anchor, the boat was reasonably stable though not the rock solid platform Nekton likes to portray. There was some movement from swells which would appear unavoidable. I'd say it was better than a typical hulled craft.
Underway however, in the passage from Puerto Rico to St Croix, it was NOT the stable platform. Look at it. You're driving a building around. It's inevitable that you're going to be bobbing around, even though they do their best to ballast. On the way out and the way back, we were seriously bobbing around.
I'll note an important point: The position in the boat did have some effect. Up and down and front to back. We were at the bottom which I would theorize reduced the movement compared to the higher story. We were in the middle of the boat until the sewage overflowed into the bathroom and they moved us to the front of the boat. I felt the front was better. I really felt the trough but particularly the lift when I was in the middle. The zoom up was the most effect on my stomach though I never actually got sick. I find seasickness is part mental, so I kept busy and didn't get sick though the trip back had considerable movement. I was trying to complete packing but finally had to just lay down.