Answer: Damp weather is ideal for fungal problems to develop, and you may be seeing something such as anthracnose on the zucchinis -- this would cause sunken spots on the fruit. It occurs in warm rainy weather, can be seed borne, or can survive in the soil and on plant debris, so it is important to rotate your crops and allow adequate spacing between plants. On the other hand, they may simply be waterlogging if your garden is poorly drained. Another possibility is squash bugs have damaged them and rot has set in as a result.
Remove the infected fruit as soon as you see signs of the problem. Avoid overhead watering, and replace the mulch immediately around the plants with a fresh layer. Destroy the old mulch and plant debris to try to limit reinfection.
Bitterness in squash is generally due to fluctuating soil moisture. Water thoroughly as often as needed to keep the soil moist but not waterlogged.
Hope subsequent fruit is abundant and tasty!
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