When sailing press and hold the right mouse button. If you've spent the skill points in them, you'll have icons for "Left Broadsides" and "Right Broadsides" these shoot out straight from either side.
When purchasing ships you can see how many broadside guns each one has. I would get whatever ship has the most you can afford, and go out sinking Navy ships! Find one that doesn't move, shoot one side, spin around, shoot the other. Did I mention the part where they don't even hit half the time? If the ship is lvl10 or above, it can take 15-20 mins to sink this way.
This is the ONLY way to raise it, and yes, it is veeeery slow going. Let's hope they tweak this soon!