Physical Water Surface - can't get imported custom models to float on the surface

I’m having issues with floating my imported custom made 3d models at the water surface level (created from 3d scans, obj files). They seem to float well below the water line.

I’ve tried resetting the pivot positions into the centre of each model and changing the float height - neither makes any difference.
Can anyone point me in the right direction?

p.s. pls excuse this novice.