Native English speakers, could you please help me with these issues?

Grammatically it is correct

but "retrieve" is a poor word choice.

Better choices:

- return

- move

Slightly less good, but still OK

- restore

- remove

retrieve cannot be used with to.  retrieve means to obtain again or recover from (the motion is toward the self).  You could, conceivably, retrieve the whales from something, like, say the mud.  But you cannot retrieve them TO somewhere.

You would be expected to return them to the ocean.  You might need to retrieve them from the beach, but after you do, THEN you send them somewhere.  Normally, we do not think about retrieving whales from a beach because we are not truly obtaining them, and we almost certainly are not getting them back (if we used to have them we could get them back after they were lost from us, but we almost certainly never had them before so cannot retrieve them, as we never had them).

Normally, we save them from being stranded, which might be talked about as recovery, and then return them to the ocean (to where they came from originally).

No, it's wrong, It should read 'when whales get stranded on the beach, it's difficult to return them to the ocean because they are too heavy.