Bring a diaper (nappy). When you go swimming, you can put valuables in the diaper (cellphone, jewelry, etc) and not worry that anyone will take it.

• beer (cheaper away from the beach)
• hat for shade
• fishing pole if you like fishing
• cheap old phone if you have one, can swap your sim card to it. Keeps sand and water away from nice newer phone

Trashbag, maybe not for what you bring but for the bits of crap you’ll find scattered around the beach

Slippers.

Also put some ice on your water bottle; it’ll keep your water cool.

I’m not going without a book/ereader.

And multiple cameras.

(Edit) particularly for the reader, I should mention “be careful of sand” and maybe have one of those little duster cans around to make sure the ports are clear. But it’s too much of what I value in between soaking in the water to not bring.)