The aperature ring started getting a bit difficult to turn into/out of f/22 about a month ago, no idea why. Well before the beach trip. Sand is the suspected culprit for a slight gritty feeling on one end of the focus ring that manifested immediately following the beach trip.
it's probably dirty, that is the first culprit
another might be that the detent spring might have been damaged (unlikely).
lease likely is something getting into the iris assembly. you will never know until you open it up
it's like pain when you pee. it can be a lot of things, specially if you are a bad boy
for people with kidney problems like me, just drink more water