Belugas have thin skulls, so a good hit with a high velocity bullet will be immediately fatal. But if you intend to use a high velocity rifle loaded with expanding bullets, the trick with belugas is getting a clean head shot; the head is clear of the water for a very short period of time. If the bullet hits the water, much of its velocity is lost and the bullet expands readily in the water, so the reduced velocity combined with the large frontal area for bullet weight makes penetration suspect. The problem then is not so much the target as the target's environment.
Although I've never seen a narwhale skull, and know little about them, they do seem to spy hop more frequently then belugas, presenting an easier target for the hunter armed with a high velocity rifle.
My approach would be a large caliber rifle loaded with heavy bullets loaded to a moderate velocity. I might even opt for solids, as there is less chance of these breaking up in the water. So my recommendation is a .458 loaded with 450 or 500 gr Barnes banded solids for both species as well as bowhead.
