The hp rating is given at some speed, rpm, that is also typically peak hp, which is not the same rpm as peak torque.
Imagine two engines a 6L and a 3L which we have magically been able to make identical in every way (all of the inefficiencies, heat losses, leakage, and etc.). If the 6L generates 409 hp, peak at some rpm, the 3L would need to spin at twice that speed to generate the same hp. In reality this does not happen for a variety of complicated reasons (e.g. the friction of a piston oscillating at twice the speed would go up dramatically). Additionally, It is far more difficult to get something to reliability run at 6000 rpm than 3000 rpm.
It is not an impossibility, but I suspect the cost of this is far too high at this point for such a low volume market. The auto industry has the massive advantage of volume which allows them to incorporate more advanced systems, engineer the cost out, and amortize that cost over the many millions of units sold annually. The marine industry, particularly the inboard marine industry, does not have this so they have to be a bit more risk adverse.
Pure speculation, but I suspect the marine engine industry relies on the pre-engineered solutions that trickle down from the automotives to make big changes. Smaller displacement engines would be a good place to start. Technically, a diesel solution would be a good one but it seems like that is an even bigger cost hurdle to overcome than spark ignition.