Allow me for a second to make the case for having 419 (or even 417) HP over the nice even 416. Why 419? Because that is the maximum HP Walrein can have for 2 turns of lefties+Ice Body to exactly cancel out the HP consumed while making a sub. You still have 104-HP subs, you have (slightly) more HP to work with, it takes proportionally less HP to set up a sub, and you take less damage by passive means. The only downside is that you have less extra EVs to spend on Special Defense. First of all, 416 is divisible by 16, and therefore divisible by 4 and 8 as well. As tempting as it is to recover the maximum percentage of HP each turn, I say that is is not completely optimal. It comes at a cost of increasing the percentage that substitute drains, as well as increasing the percent damage that all forms of passive damage do. That means that if Walrein is harmed by entry hazards and Toxic damage alone, you could possibly die by virtue of purely passive damage taking away exactly the damage it needs to kill Walrein. If instead you opt for 419 HP, you could be left alive with 3HP after the end of this unusual onslaught.
In another scenario, you bring Walrein in on a physical attacker that deals out from 312-314 damage. If you had 416 HP, you would not have enough HP to set up a sub. However, you can still set up a sub if you had 419 HP because of the 3 extra points. Literally all that the 3 extra hit points do is add to the physical durability of Walrein. It does not affect anything else. It may seem like I am splitting hairs, but this is more of a matter of making the EV spread the very best it can be.
TL/DR: 416 HP is good, but 419 HP is better
Just my pointless rant for the day.
Edit: Some interesting calcs.
+0 Modest 252SpA Cobalion's Focus Blast vs 220/36 Walrein (416HP): 85.10%-100.00% (2.56% OHKO)
+0 Modest 252SpA Cobalion's Focus Blast vs 224/32 Walrein (417HP): 84.89%-99.76%
+0 Modest 252SpA Cobalion's Focus Blast vs 232/24 Walrein (419HP): 84.96%-100.24% (2.56% OHKO)
+0 Timid 252SpA LO Azelf's Grass Knot vs 220/36 Walrein (416HP): 69.71%-82.21%
+0 Timid 252SpA LO Azelf's Grass Knot vs 224/32 Walrein (417HP): 69.54%-82.01%
+0 Timid 252SpA LO Azelf's Grass Knot vs 232/24 Walrein (419HP):70.17%-83.05%
It appears that Walrein is slightly more bulky on the special side with 224HP/32SpD (417HP) than with 220HP/36SpD (416HP). There is now literally no reason to use 416 HP.