Coaxial cable in IT Earthing System
How can two systems or an antenna in a ship be connected using coaxial
cable and not introducing direct or indirect connections between
supply lines and earth?

The simplest method is to use isolating signal transformers but there are isolating amplifiers as well that can be considered: -

For antennas: if the antennas are operating at a high enough frequency then the shield on the coax can be capacitively connected to the driving circuit (or receiving circuit) without much chance of problems. Less than 100 pF is about as much as I'd dare consider without details.