With 4120Kv, it's a 'typical' 2S set-up giving you 30.000 rpm unloaded.
On 3S that would result in 45.000rpm unloaded, which is rather hard on the bearings of both motor and drivetrain.
Also metal props would be mandatory, as the plastic and carbon filled ones will shed their blades...
The 36 mm prop if fitting for the hull size, going smaller will result in an 'endless' run untill the boat gets on the plane...
Not my favourite option.
A metal Ø36 mm threeblade, or a slightly larger two blade will be more efficient.
Something like this: