You are not alone. That's just rude & inconsiderate. I usually bite my tongue when this happens OR politely ask them to turn it down a bit. What peeves me even more is when they leave the music playing & go into the water for extended periods of time. This is one of the reasons I don't go to the beach here in FL. People think only of themselves, not those around them. I want to listen to reggae, not have someone else's choice of music inflicted upon me. That's why there are earbuds.