Final answer:
The complement in the sentence 'Our first island adventure was a visit to a coral reef.' is 'a visit to a coral reef', which provides more information about the subject 'island adventure'.
Step-by-step explanation:
In the sentence, 'Our first island adventure was a visit to a coral reef.', the complement is 'a visit to a coral reef'. The subject of the sentence is 'Our first island adventure'. In English grammar, a complement is a word, phrase, or clause that is necessary to complete the meaning of a given expression. Complements usually follow linking verbs like 'to be', 'seem', 'become', etc. In this case, 'a visit to a coral reef' is a noun phrase acting as the predicate nominative, which means it is renaming or providing more information about the subject 'island adventure'.