A water pump was something I always wanted as a child. Because I never got one, I had to design a simple pump that hopefully makes some other kids happy.

1. Print the base plate, the cover, the cylinder, the handle and two rings.
2. Cut the diaphragm from a rubber balloon.
3. Cut the valves from a thin sheet of polypropylene or similar plastic.
4. Take an M6 bolt with a length of at least 65 mm and a thread length of around 20 mm. Saw off the head of the bolt and glue the handle to the non threaded end.
5. Slide the cover over the bolt and place the following components one by one onto the bolt: nut, ring, diaphragm, ring, bolt (see section view for correct orientation). Tighten the nuts.
6. Insert 8 M3x40 mm bolts into the base plate, slide the valves over the bolts onto the base plate. Next slide the cylinder over the bolts, followed by the diaphragm and the cover. Attach nuts to M3 bolts and tighten them.
7. Attach hose to in- and outlet.

The attached Creo Elements Express file can be used to modify the parts for your needs, particularly for other hose diameters.

Update:
The pump works but watertightness is an issue. I had to add thin cardboard seals between the parts.