Can they actually understand each other using echolocation though?
No, their communication vocalizations and echolocation sounds are different, although both animals do communicate through sound. Of course, sound is propagated much further and much faster through water, so it sounds totally different.
Of course, Friends break all kinds of normal animal rules, so who knows?...
Might "I love you too" or likewise be worth using for the third panel's translation? In my opinion, "me too" is a very literal translation of the original Japanese "ワタシモ" which doesn't form a natural reply to "I love you" as an English statement; I feel the context provided by the second panel suggests "I feel the same" is understood moreso than literally "I as well".
