Mother's warning as boy, 10, suddenly falls ill with potentially-fatal condition after swimming at New Hampshire beach

A New Hampshire mother is sounding the alarm after her 10-year-old son suffered hypothermia while on a trip to the beach. 

Declan Cassani, 10, said he 'saw black' moments before he threw up and passed out on Hampton Beach on the Fourth of July. 

His mother Heather told WMUR that he was playing in the water all day and warned that the symptoms of hypothermia struck him suddenly and out of the blue. 

Lifeguards cautioned that although much of the US has been under scorching temperatures this summer, families should be aware that oceans often remain freezing cold. 

Declan Cassani, 10, suffered hypothermia on the Fourth of July after spending the day frolicking in the ocean at Hampton Beach, New Hampshire 

Heather said she is sharing her son's medical emergency in the hopes that other families can avoid a similar fate. 

'That's really what it's all about because you never know what all the things that could go wrong and you try to protect them, but it's like I feel like knowing is half the battle,' she said.  

Although Declan appeared normal throughout the day as he went in and out of the water, his first symptoms set in quickly as he suddenly became disoriented and confused. 

Declan passed out and vomited, and he recalled that 'all I saw was like a black screen - and then I fell.' 

'I didn't know what was happening, and I just got scared,' he said. 

Hypothermia is a condition caused by the body's core temperature dropping, and can lead to drowsiness, slowed heart rate, unconsciousness and death. 

Declan's mother Heather said her son was playing in the water all day, and warned that the symptoms of hypothermia struck him suddenly and out of the blue

Other beachgoers spotted the medical emergency and called 911, and lifeguards spotted early signs of hypothermia as Declan's skin was cold 

Hampton Beach Chief Lifeguard Pat Murphy warned that although summer temperatures are high, they do not always translate to warm ocean waters, and warned that 'hypothermia is something we see quite a bit'

Lifeguards and other beachgoers raced to his side after spotting the medical emergency and called 911. 

First responders found Declan under an umbrella, and spotted signs of hypothermia as his skin was cold. 

He was moved into the sun and was said to have recovered quickly after warming up. 

Hampton Beach Chief Lifeguard Pat Murphy warned that although summer temperatures are high, they do not always translate to warm ocean waters. 

'This is the coldest we've seen the water last this long,' he told WMUR. 

'So, you go from a day of being 95, 85, to water that's 50 low 50s and you stay in it for a long time, hypothermia is something we see quite a bit.'