A cloche is certainly worth trying, though I wonder if you have vine weevil grubs eating the roots. They're c- shaped White grubs with brown heads. If feasible, it might be worth taking the plant out ( being very careful not to damage the crown) and having a look. If you find them, I fear there's not much you can do, the recommended chemical cure isn't safe for food crops. I always water my cucumbers in pots from underneath, putting them in a saucer and watering into that. If they haven't slurped up the water within half an hour or so, then I empty the saucer. It doesn't sound as though your plant needs any more water at the moment though if the compost is wet.