Well... The impact energy calculates 1/2.m.v^2... (m is the mass, v is velocity)

That means that a cubic meter of water traveling at 2 m/sec (7.2 km/h) results roughly in 2000J or 2000 kg.m^2/sec^2 of kinetic energy which gets transferred fully onto a solid, immobile object...

A sailor standing on the foredeck is now not immobile, which results in the object of say 80 kg of mass getting accelerated to the speed of the wave because the mass of the man is rather small compared to the mass of the water...

That in result means he will be pushed into the tethers by 2100 J or 2100 kg.m^2/sec^2 plus additional pressure from the water still pushing the sailor...

If the tether now has no stretch, all of that energy is transferred via the harness to the sailor in an instant... If the man is stopped in a tenth of a second the force on the harness and therefore the sailor equals to something of 21 kg.m^2. Now divide this with the area of the harness in m^2 and you should get the actual pressure on your body from the harness in kg...

I hope i did not make any mistakes here...

I think you have been right, the forces are really not that high as i initially thought...