With a 12v circuit feeding 5x 300w inverters over a distance of 15meters. What cable size should I use?
Lets say, no more than 0.5V drop, 130A, 30m.

Rule of thumb for cable size:  

\$Q\text{[mm}^2]=\dfrac{I\text{[A]}×l\text{[m]}×0.018}{\Delta U\text{[V]}} = \dfrac{130×30×0.018}{0.5} = 140 \text{ mm}^2\$

Result: at least \$140\text{ mm}^2\$, and you need two of them. Thats about 13mm diameter without insulation, and you need two of those. You also need to attach to that cable at each cabin. 

You should put the inverters near the boat´s power source and transport the high voltage over distance.

Another option would be to put an appropriate AC generator on the boat and keep the power supply completely off the Ship´s DC system.