I usually go after work, so the water has been warming up all day, the crowds have mostly gone, so I can find a parking space. No need for sunscreen then, either. There are usually still people about. If there are no swimmers around, I'd be wary of going in.
My bag has two towels, cossie, an old sun dress, swim shoes for shingle beaches, goggles (don't always use them, especially when trying to avoid ears getting wet.) Hairbrush, water bottle.
You don't need all of that. Towel and cossie. I leave my keys in my bag on the beach, as the swim bags I've tried haven't been waterproof when testing.
In the last 10 days or so, I've swum at Southsea, Boscombe, Calshot and Weymouth, fitting in with theatre and friend visits. You can check water quality online, and also sea temperature. It's lovely right now.