The "Kaijû" in the Japanese title is the Japanese term for a marine mammal, written with characters that mean literally "sea beast". It is pronounced exactly the same way but is a different word from the "kaijû", written with characters that mean literally "strange beast", that refers to (usually non-folkloric) monsters such as the titular characters of "Godzilla (1954)" and "Mothra (1961)".
Joe Hisaishi's scores for this movie and "NiNoKuni (2019)", also set to premiere in 2019, are his first for a non-Studio Ghibli animated feature since "Venus Wars (1989)", which was premiered almost exactly 30 years and one month earlier.
When the movie was first announced to the public in February 2019, Airu Kubozuka was announced as the voice of Sora. However, in April 2019 it was announced that Kubozuka had dropped out of the role because his voice was in the process of breaking, making it impossible for him to voice the character consistently, and role had instead been taken on by Seishû Uragami.