The boards are surprisingly stable.  There are two main factors that attribute to the stability of these bikes.  The board dimensions are larger than a regular paddelboard (13'x3') and there is also a rudder that extends 10" below the surface of the water to aid with stability.

The water bikes are very stable.  There is a group that has riden around the waters of Antarctica which speaks to the stability of these boards.  If you do try to test the limits of these boards and happen to fall off there are multiple grab handles on the board to assist with leaning the bike back up and either getting back on or pulling the bike to shore.

The resistance is based on the propeller size you choose to use.  Different models offer different propeller options.  After a brief acclimitization to riding on the water the experience is "just like riding a bike" but better!

I have two girls who love riding them! My girls are 8 and 10.  Although they are still too short to sit on the seat the top tube is quite wide and they are happy to sit on that and pedal away.  Riders taller than 4'10" would likely be able to sit comfortably using the bike seat.