Very strong constant magnetic fields don't seem to be dangerous (at least at the kind of levels MRI machines work at, which are within an order of magnitude of the strongest magnets on the planet). A very rapidly changing strong magnetic field can be dangerous, because it induces currents in your body, like in your nerves. There are safety limits (very conservatively set) in MRI machines on how quickly they adjust magnetic fields, as if they were to really go for it there's a risk of heart problems (in the moment, it's not like CT scans where more exposure means more risk later in life).

We don't have any evidence they are dangerous. There are some phenomenon like magnetophosenes. You could also look up what a magnetar would theoretically do to a body, but nobody is getting close to one of them anytime soon.

not at all whatsoever. however if you have gadolinium injected for contrast some people have problems with that. but the magnetic field itself doesn't affect you negatively on its own

Some experiments with magnet, water, blood etc.

https://www.youtube.com/watch?v=E-yHv91Y_oI