Dear Søren,
Other than the wave period (first column, in seconds) and the wave direction (2nd column in the .3 file, in degrees), the values in the WAMIT .1, .3, and .hst files are nondimensional. The added mass is normalized by density, rho, the damping is normalized by rhoomega, the wave excitation force is normalized by rhogravity(wave amplitude), and the hydrostatic restoring is normalized by rhogravity. These normalizations also include length scales; however, all of the WAMIT models we’ve created are based on a length scale of unity (L = 1). More details can be found in Chapter 4 of the WAMIT User’s Manual: wamit.com/manual6.4/Chap4.pdf.
I hope that helps.
Best regards,