Over watering can cause chlorosis. The picture showing the yellow leaf with green veining looks like chlorosis. I would purchase a bag of potting soil and replant the zucchini into the new soil. Potting soil is good for container gardening because it is lighter than garden soil.
Set the repotted plant under an overhang if you are going to get rain. It needs to be damp, but not wet. When the plant seems okay again, you can return it to its regular spot, and everything should be fine. Remember to water at the soil line and don't get the leaves wet. Wet leaves invite powdery mildew.
If you find it hard to tell when to water, Burpee makes an inexpensive water meter with a probe that gives you a reading regarding soil moisture.
Spread your old soil on a tarp and let it dry. Then it should be fine for future use..