Why we need big propellers at slow speeds
The push you get from a propeller is proportional to the change in momentum the propeller gives some water which is: mass * deltaV
where deltaV is the change in velocity of the water. The energy put into that water by the propeller is: 1/2 mass * deltaV^2
If a boat is going fast a propeller passes through lots of water and so can push a little bit on many different cubic feet of water and so only change the velocity of any given bit of water by a small amount.
A propeller that makes efficient use of 50 Hp at 15 MPH makes very inefficient use of the same power at 2 MPH. At slow speeds it goes through less water and gives it a much higher deltaV.
An efficient propeller for a slow seastead that will be large so that it can give a small deltaV to a lot of water. Using a small fast propeller on a big slow seastead would take far more energy for the same push.