I hope I can help you here. Please understand that no online can state anything with certainty, all I can do is give my best educated guess based on what you wrote.
It is normal to have a red mark when the skin is irritated, or hurt but not punctured. Whatever you stepped upon made a mark, but the mark eventually faded. Your skin was not punctured, it was intact.
IF you did not break the skin, there is no chance that you could get HIV, no matter what it was you stepped upon. Intact skin acts as a barrier to the HIV virus.
From 2006, an online HIV resource: http://www.hivmedicine.com/textbook/pep.htm
It essentially says that on intact skin, the risk of transmission is negligible to virtually nothing. "exposure of intact skin to HIV-contaminated body fluids (e.g. blood) is not sufficient to transfer the virus. "
Not only was your skin intact and unbroken, there was not even any blood, whether it was your own or anyone elses. And, had there been HIV virus, HIV virus dies pretty rapidly in room air, so, it would not have lasted long at all in the cold water and sand at the beach.
BotXXXXX XXXXXne, I cant state 100% for sure online that anything is so, but I am pretty certain that you have virtually nothing to worry about.
I hope that eased your mind and calmed your worries,