The number of turns a motor has refers to the coils of wire that are wrapped around the rotor.
Generally, the more turns a motor has, the more torque it will generate and less RPM . Less turns equals higher RPM, less torque. It also equals higher current draw.
The diameter of the wire used for the windings "Turns" also has a bit to do with the power output of the motor.
Some motors also use multiple windings. This is often seen as a something like an 11 turn 2 or 11 turn double.
If you are going to more powerful motors you will want to look at your speed control very closely. It may have to be upgraded to handle the extra load the new motors will put on it.
The drive system on the boat may also have to be re-done to handle the extra power.

Peter