This is hard to diagnose. Because it is a common disease and because it causes irregular holes in the leaves I am inclined to suggest it is angular leaf spot casused by the bacteria Pseudomonas syringae pv. lachrymans.
Another bacterial leaf spot is Bacterial Leaf spot caused by Xanthomonas campestris.
This from the APS Compendia on Curcurbits https://www.apsnet.org/apsstore/shopa...
"Symptoms may appear similar to those of angular leaf spot. On cucumber, small, watersoaked lesions first appear on the lower sides of leaves and are later visible as indefinite yellow areas on the upper sides. In about 5 days, the lesions become round or angular with a thin translucent center and a wide yellow halo. The lesions enlarge, reaching diameters of up to 7 mm.
Symptoms on winter squash, pumpkin, and gourd leaves can easily be overlooked because of their small size. As lesions coalesce, they may appear similar to those caused by Pseudomonas syringae pv. lachrymans."
I dont think it is fungal.
Another possibility is that it just mechanical and you recently experienced a hail storm? But I guess that is not the case
What to do? Remove affected leaves and dont spray water over the leaves
If you want to use chemicals then one recommendation is copper sprays. But APS Compendia above says that is ineffective once the epidemic has begun (page 33).
can you get any closer up images and inspect around on the healthier leaves to see if anything is beginning