The Diving Suit (aka Super Diving Mask) is an equipment that heavily reduces oxygen consumption and increases movement speed and cold protection of the player in water. This allows players to be able to stay much longer in the ocean (and lakes). It also gives the same protection as a gold helmet. Note that when on winter or forest, the Diving Suit won't give cold protection.
Despite the tooltip, you swim at the same speed as with a normal diving mask. Players with diving gear that are holding weapons swim almost as fast as players without weapons or diving gear equipped, making savages with diving gear extremely dangerous to those without it.
A normal Diving Mask multiplies your oxygen in the water by 2.27. The Diving Suit multiplies your oxygen in the water by 4.17.
Without diving gear, you will lose 5 temperature units during the day, and 25 during the night. Wearing a Diving Mask reduces this to 3 and 10. Wearing the Diving Suit reduces this to 1 and 5.