Transient Voltage Suppression diodes
Load dump is based  on load during disconnect.  Power to dissipate existing load may be quite  small unless you have a big boat load of accessories.

So it depends. Normally use Joules of stored energy in inductive  load. .  Even a big cap on 12V may resonate. 

so, energy = Joules = Watt-sec.  

Power in inductive load (trolling motor?) =W?

1st approximation is quench the same current as the worst case load during disconnect and duration depends on stored energy in a system with a disconnected battery. 1/2 CV^2 and this energy will resonate with open circuit inductance of powered devices but get dumped into TVS.  Intermittent contacts giving the worst case condition.


  Time duration depends on storage capacitance of loads and resistance.
  So the exact load dump depends on the load, not the ISO spec for
  industry standard applications.  Your app. is non-std.


Either measure or guess . My guess is you have a small boat, so just ignition and low inductance wiring. 

1ms should be fine.  (1/1000us would be my choice of test condition)

Biggest category is for AIr conditioner clutch in a car.

More details here from ON semi.  http://www.onsemi.com/pub_link/Collateral/AND9098-D.PDF