I remixed the Balloon Boat project by Keye to make this fan powered jet boat.
I removed the balloon attachment and have used a 30x30mm fan and two AAA batteries to provide the air. The small circuit board you can see is a voltage converter which raises the 3v from the batteries to 5v to give the fan enough speed to power the boat.
The fan is glued into place and I have filled the screw holes in the fan with Blu Tack to ensure all the air from the fan does not leak upwards rather than down the hole to power the boat.
I've left a couple of Lego pins at the front of the boat for a minifigure captain.
Thanks also to mrbenbritton for the battery holders I used for this project https://www.thingiverse.com/thing:119847