Would "se puede ver" and "se pueden ver" both be correct Spanish?

Question

I think I can understand why "... se puede ver a las ballenas jorobadas retozando, coqueteando ..." is correct - [meaning: "... one can see the humpback whales ...", or "... you can see ..."] - after reading your Kwiziq lessons " https://spanish.kwiziq.com/my-languages/spanish/view/6933 "  [number 6933] and " https://spanish.kwiziq.com/my-languages/spanish/view/5132 " [number 5132].However - under 'Your Practice', you recommend the lesson " https://spanish.kwiziq.com/my-languages/spanish/view/6089 " [number 6089] - which seems more consistent with this answer: "... se pueden ver las ballenas jorobadas"? (i.e. with 'pueden' in the plural, and without the "a" [before the 'las']). Would this^ be a correct way of saying "... the humpback whales can be seen ..."?

Answer

Hola David
Interesting...
This is one thing the grammarians don't seem to always agree on.
When this structure is used with a modal verb using the passive se, this type of ambiguety comes up. You can say both:
Se puede ver a las ballenas jorobadas retozando (using singular in the verb and plural in the object. The lack of agreement would make this an impersonal sentence, normally translated in English as "one can see..." 
But we can also say:
Se pueden ver las ballenas jorobadas retozando (it would sound very odd/incorrect to use "a" here, with the verb in plural) - This would be seen as a passive se sentence "the whales can be seen..." 
This seems to be a sentence that fluctuates between these two types of sentences. 
It's not easy to find a solid article about this, I guess, because of its ambiguity.
I found some information here where they try to explain different cases of passive se and impersonal se sentences. Sorry it's all in Spanish but I'm sure you can get an idea. It starts mentioning this case with the preposition "a" on the third paragraph. I hope it helps. 
Some info about passive se and impersonal se sentences
Saludos cordiales y Felices Fiestas.
Inma

Answer

Thank you Inma -In this particular case, it would probably be better, under "Your Practice", to point us towards Kwiziq Lesson ".... 6933" [or .... "5132"] - see above^, rather than ".... 6089", so as to correspond with the answer given in the Dictation.But we do really appreciate this facility of being able to ask questions. I remember that somebody once commented that you have the 'patience of a saint', and they are quite correct !All the very best for 2022,David M