And in fact, I've done just that.
First of all, since water has a mass of 0.03, and each block is 1m on a side, and 1 m^3 of water has a mass of 1000 kg, one unit of mass in StarMade is 1000/0.03 kg = 33,333.3 (repeating) kg.
With that in mind, I created a ship with a mass of 300, or...