I'm always wondering what cause someone who replies with this is trying to advance.
You are probably correct, we don't know what the cause is for this specific problem. We do know that we cause massive, let's say, "alterations" to the oceans by acidification and plastic pollution alone, and there is very strong indication that the effects of those alterations are not positive.
So the worst case when addressing those issues is that we have solved or at the very least ameliorated acidification and plastic pollution, only to find out that gray whales dying specifically was not covered by those causes.
Yes, finding out the actual cause is important (there may be other issues we are not yet aware of), but prioritizing the recovery of the ocean in general is not exactly a waste of effort in the meantime. As an analogy, if a bathroom smells like excrement, fixing the toilets known to overflow is a suitable activity. If it still smells like excrement after, or if you discover a different cause while the toilets are being fixed, you're still glad you fixed those toilets.