This boat engine implements the principle of the pop-pop boat mechanically. Water is drawn into and expelled from a pressure chamber by means of a diaphragm moving up and down periodically. I mount the engine on a styrofoam "hull" and corrugated plastic "deck". The boat can move with speed about 15 cm/s. Notice that this engine has no underwater moving parts, the diaphragm is located above water level and only pushes and pulls trapped air inside the chamber.

The diaphragm here is made from silicone rubber cast on a 3d printed mold, but other materials could work, like plastic bag or umbrella fabric, maybe with silicone caulk for sealing. The power source is a 130 dc motor running from 2 AA batteries.

I printed the chamber with support material included in the stl file (vertical clearance 0.5mm) because automatic support blocks the outlet hole. There is also a version without support.

The second gear is printed 2x and have plain 3mm shaft hole. The third gear must be locked to the shaft by tapping and placing a nut against it. The shaft protrudes out to hold the crank, which is locked the same way. Anyway, there are 2.8mm and 3.2mm hole diameters, all the 2.8mm holes are for tapping and others plain.

Video of the boat: https://youtu.be/MhlXEtJgdNg