The “optimal” temperature for swimming is around 78 F, though the average surface sea temperature in the northern Pacific Ocean ranges between 45 and 61 F, while the northern Atlantic Ocean is a few degrees warmer. Water temperature below 50 F is considered “extremely dangerous” because it saps your body heat and drastically increases the risk of hypothermia. It’s recommended that you use immersion wear (wetsuit, dry suit, or even scuba gear) if entering water colder than 60 F.