Er… i believe the temperature of water (even in abyssal plain) will never fall below water freezing point. And i believe it’s about 4 degrees Celcius. Anyway, it’s cold enough for most of oceanic animals, so most of them are adapted with that low temperature by using either freeze-tolerant or freeze-avoiding mechanism that involving specific anti-freeze/cryoprotectant substances like polyols (glicreol, sorbitol) or AFGP (Anti-Freeze GlycoProtein) or AFP (Anti Freeze Protein) to prevent freezing. Also the animals that live in the abyssal plain need to maintain an extremely strong internal pressure to withstand the water pressure in that depth.