Answer:
Surely its endothermic poikilothermic. The walrus would use to much energy trying to maintain a stable body temperature during a resting state. To save it from having to use massive amounts of energy in a energy restricted environment, the walrus would have some level or variation of body temperature. Also something to think about is ocean water, when it is say -60 air temp outside, the water temp would still be close to (if not frozen) about 33-36 degrees or so. The walrus could actually use the water to help itself to warm up, since it is actually warmer in the water than the outside air temperature.