I swim the sea several times per week. There isn't too much of a sewage problem in these parts but I always check the water quality anyway and if it's in any way iffy, I go to a local reservoir instead - tbh, at this time of year, I'm more concerned about Lions Mane jellyfish, which are blooming in large numbers so I tend to swim in freshwater in July & August.
Luckily my local beach has hooks for belongings. If I go elsewhere, I take a camping chair for my stuff so it's off the ground - easier to get changed if things aren't all sandy and it reduces the risk of a dog peeing on it, or an over-excited child running through and inadvertently dragging my towel into the water.
I rarely swim with other people - I like to actually swim and find that most groups are about dipping/bobbing - no shade here, that's absolutely fine but not what I want. Where possible, I like to have DH or my mum on the beach (can't persuade them to come in) to look out for me. If they are not available, I will go in only if there are other swimmers around. I find that, whether bobbing or actually swimming, folk tend to keep an eye out for each other. If it's deserted I don't go in.
As with others, I use a dry-bag tow-float for phone, keys etc, and also for visibility.