This will appear to be a magic ring, made of gold, and with some sort of ocean-themed imagery on it (sea shells, fish, octopuses, etc.). Any character entering water will find they can breathe it as normally as air. In addition, the wearer feels no ill effects from water pressure as they swim downward. The character gains extra mobility, being able to move through the water as easily as through air, and can see underwater as clearly as above it (though darkness effects still apply as normal). There is no apparent time limit on this; the character may use it as long as they desire.
The curse comes in when they try to take it off. The character will be unable to remove the ring; what's worse, they'll find they can no longer breathe air, but can only breathe water. Walking around on land will require some sort of water supply. (Perhaps specially modified armor, filled with water?)
Either way, it will take a powerful Remove Curse spell to get the ring off, unless the character severs their finger. However it happens, the character returns to normal once this occurs. The ring, itself, remains cursed.