Red spots on penis

Hi, last weekend I noticed some red spots on the tip of my penis. i spent the day in and out of the sea and pool and thought it was a reaction to the mesh on my swim shorts. i washed daily and it disappeared. yesterday i went swimming in the sea only and spent the day in my mesh shorts again and when i got home the red spots were back. could this be a coincidence or is the ocean or my mesh shorts making the rash? it is not itchy or bumpy yet, but i can feel it. see photographs, please help!!

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Work out which one by either swimming in the ocean with different shorts (without the mesh if possible). Another way will be to use the same shorts out of the water.