A hypothetical question of what is technically more efficient in the case that the propeller has 20% too much pitch:
a.) Change the gear from 1.36 to 1.6?
b.) Change the pitch from 28 to 24?

Ayuh,..... Change the prop,..... Changing gear ratios is extremely expensive,...

"Technically more efficient"? One could make an argument that changing the gears is more of a 'fixed' or permanent "hard" change...as props have SO many variables. In a solid medium, NOT water... the 24" pitch and 1.60:1 ratio is just under 1% MORE efficient, i.e. it would advance 17.64" vs 17.50"...from a mathematical perspective.

Bondo, I know.
The question is hypothetical so I can better understand the "problem".
By changing the propeller, you keep the high rpm (3.800 rpm for example) and the same torque. Due to the reduction in pitch, you get a reserve of power for prop, but the engine needs the same power-torque in relation to the diameter of the propeller.
By changing the gear ratio, you change the rpm and increase the torque on the propeller. It seems to me that in this case the engine uses disproportionately less power to rotate the same propeller diameter and you can increase the pitch much more and in the end get a faster speedboat. (Everything, of course, within normal limits and taking into account the weight of the speedboat.)
While looking through old posts, I came across information that Fountaine sometimes used a 1.65 gear, which led me to think this way.

Example from air prop for models Ecalc:
Motror HET 1w35 and 5s LiPo 2.700 mAh

a.) with 3:1 gear and propeller 8x5
- power 832 W
- static thrust 6.8 lb
- pitch speed 102 mph
- thrust 2 kg at 62.5 mph

b.) with 5:1 gear and propeller 8x17
- power 839 W
- static thrust 8.8 lb
- pitch speed 212 mph
- thrust 2 kg at 137.5 mph

I know, air it is gass, but water it is liquid, but?