Handy,
The electric motor by design ( squirrel cage induction motor ) is a horrrible start-up current sucker, no matter if you free wheel the compressor. You will free wheel anyway for a dive compressor, I mean open the water seperator is like free wheeling already.
It is not about mechanical load of this SCIM design that caused the 600% current rush, its the way it is designed.
If you free wheel, the current rush magnitude is the same but the duration is so short, unless you have a very good fast sampling data logging current meter, you will see only 300% start up current but in fact for a very-very brief period the 600% exist but not recorded. You don't even need to put on the belt, and run the motor "naked", it will need what it will need by design.
By having a load on it, the duration of the current surge will be longer until the motor hit 80% of its design RPM and it will dip down and eventually when its running at 100% designed speed, the nomimal current rating will apply but less than that actually because people usually rig an electric motor to a compressor or whatever mechanical load choosen at 80% motor capacity.
The motor itself usually will not have a 1.0 power factor for 3 phase model. Usually 0.8PF is common but three phase motor does not need starting capacitor, which sometime is the failure point of a single phase motor.
Your 20Kw should be sufficient power wise if you choose the motor wisely like Craig advised.
Have fun.........
IYA