There are two areas where electric boat motor makers get to exaggerate the 'equivalent horsepower' of their motors beyond a strict units conversion: out-of-the-hole torque/acceleration, where electric wins over ICE, and generally lower prop RPMs for electric, allowing for more efficient propeller design. As an example, the Propel S1 claims 73.5% efficiency from electric-power-in to propulsive thrust from the prop. At 10kW electric power in, that's 7.35kW of propulsive thrust, which is in the neighborhood of a 20hp ICE outboard. ICE outboard propeller designs are nowhere near that efficient due to much higher prop RPM.
My 6kW electric outboard claims 9.9 'equivalent horsepower', but math puts it at more like 8hp. Then the more efficient propeller design (due to lower prop RPM) results in 3.42kW of propulsive thrust, which is in the neighborhood of a 10hp ICE outboard.
For comparisons between electric and diesel, the prop RPM issue will likely not be as much of a factor, due to lower typical diesel engine RPMs. Then reduction gears can completely negate that advantage, but of course that adds drivetrain loss.
My guess would be that to adequately replace a 20hp diesel motor (inboard), and keep the same prop and prop RPMs, you'll need more like 15kW electric motor output. Depending on where the prop RPMs are starting, you could gain some performance by targeting lower prop RPMs and using a higher diameter and pitch propeller. Again depending on what your prop RPMs are now, you might be able to get to a 30hp 'equivalent horsepower' performance level from 15kW electric motor output. But probably not much more than that.
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