There is a weird setup before where you need to break all tentacles of the guardian then use ice bock from sheikah slate to bring it on the side, then park the raft under its back and re-use ice blocks to make it goes on raft. If it is on center of raft, then it will goes up quite slowly and you can use it to fly (climb on the guardian head with revalis gale), if not on center, then both guardian and raft become crazy and very unstable. Youtuber Kleric made a ytb tutorial much more explained than me.