Cucumbers and watermelons need to be pollinated just like squash. Peppers and tomatoes have male and female on the same blossom. They need to be pollinated by an insect or you can use a vibrating toothbrush on a petal for a few seconds to shake the pollen and pollinate the flower yourself.

It won't wash the pollen away. It might reduce bee activity and therefore fruit set. More of an issue might be brown rot, jacket rot, blossom rot, or something similar. A fungal infection of the small developing fruits. Not the apples but cherries. And not those just those blooming now but earlier blooming trees. This is most severe during prolonged wet weather, not just a couple days. UC Davis has info on these issues.

Sour cherry trees can pollinate sweet cherry trees. I'm not sure if they make the best pollinator, but they are certainly capable of doing so. If you take a cherry that resulted from a blossom that was cross-pollinated here and take out the seed, that seed can grow into a hybrid tree, which can eventually produce fruit of its own (I know that in the case of the hybrid cultivar 'Mesabi' it's even self-fertile). However, the seeds inside this fruit are sterile and cannot grow. Like breeding a horse and donkey together to create a mule, there is no third generation. The reason is a chromosomal difference, sweet cherries are diploid (2n=16) while sour cherries are tetraploid (2n=32). A hybrid between the two results in a triploid cherry, which is why it cannot further sexually reproduce. However, flowering cherry species just happen to have the same chromosome count as Prunus Avium (i.e. sweet cherries), so that means they are capable of freely interbreeding. But it should be noted the two are still not in the same species. The question then is what does this really mean? Even when it comes to the different wild varieties of Japanese flowering cherry, oftentimes crossbreeding them together results in a hybrid that grows less vigorously than its parents, or is not very well adapted to its environment. Sometimes this may become more evident several generations down the line (you might read up more on how Outbreeding Depression works). Cultivated varieties of flowering cherry, almost all of which resulted from hybridization over hundreds of years) are much less hardy than their original wild pure-species relatives. We can even see some level of graft incompatibility combining different varieties of cherries within the same species, the tree does not grow quite as vigorously or as big as if it were grafted onto its own rootstock.

Cool! Are they the same kind as the parents or new ones?