Take another look at the sentence:
In her dreams that night, Diana had __________ in a pool of warm cocoa; when she awoke, the aroma of chocolate still tickled her nostrils.
The sentence requires the past participle of swim, an irregular verb. Swum is the past participle.
|Verb||Simple Present||Simple Past||Past Participle|
Swimmed, the form you chose, is a spelling error. Before you continue, review the rules for irregular verbs.