Understanding DC-DC boost converters
Basically, the way electrical loads work is that you give them a voltage and they pull a current.  The current pulled is based on the state of the load and its environment, so things can get complicated, but in general the higher the voltage, the higher the current.

So if you make a boost converter that puts out 52V, then your outboard motor will draw whatever current it draws it that voltage and boat speed.

A typical boost converter has a closed-loop controller in it that sets the voltage.  So if you set the converter to output 52V, then under normal operation, 52V is what you'll get.  (Note that it's usually poor system design to do this when there's a DC motor in the mix -- it's absurdly easy to source a motor with a different winding, or even rewind a motor, to get more current at lower voltage.  But, rules.)