For me, it's not FPS. In a game where I can actually walk around in a space, then it's fine. The moment I use a thumbstick to move my camera, then my eyes are disagreeing with my ears about where I am. I instantly feel sick, even at the same frame rate as before.

No, it's caused by your brain and body disagreeing on movement, one thinks there is movement where the other doesn't and it's the reason you eg. in vr you see movement but your body doesn't feel it. On boats it's the reverse, your body feels movement but the brain sees everything stationary.

Yet, some people can clearly get used to such things. Boats have been in use for a lot of history...

Doesn't mean that all people can get used to it though. Or that it's worth the effort.

In a boat, you can always look to the horizon. Works for me. There doesn't seem to be a comparable "horizon" in VR.

Interestingly, I had a similar motion sickness experience the time I went skydiving.

Visual-vestibular conflict (the main cause of VR sickness) can either be caused by lag (stationary) or vection, i.e., the illusion of movement while locomoting.