A high class female boat comes to notice a rogueish, charming worker boat who is obsessed with her from a distance. They meet up once for no reason and she falls in love with him too but they have to keep their affair secret from the girl's father( a big navy boat) because that is exactly the sort of thing he hates the most.
However, it turns out that the male worker is actually a part-time submarine and the high class female boat (who just became a princess) is forced to marry a snooty, arrogant raft.
The worker boat/submarine recruits some professional wedding crashers, who turn out to be frauds, to help crash the wedding. Since they were frauds the worker boat/submarine had to confront the princess himself, but before that he has to learn who he really is. A boat, after all.
He leaves behind his submarine friends forever, and comes to the princess's aid.
When he arrives, the wedding has been postponed because her father (the big navy boat) had been kidnapped by submarine bandits. Despite commiting himself to being a boat, he dives under the ocean and becomes kind of like a submarine again. He searches for him while singing a song about love and adventure.
He finds the big navy boat of a princess's father (at this stage he is officially king) and fights off the bandits one by one using nothing but the power of love and self belief. At this point the king and him become lost for weeks but also bond during this time.
They return to the surface just seconds before everyone had given up hope and the princess was about to marry the snooty raft. The priest boat says 'Is there any reason these two shall not marry today?' and the worker boat ( who becomes a boat again forever) says 'Yes. Me!' and marries the princess.
What also happens is that his submarine friends have also become boats. The arrogant, snooty raft marries one of the worker boat's friends ( a sassy, irritating anchor, who inexplicably floats) and a stork brings them children(buoys) who start nagging their parents right away.
The big navy boat king comes over to them with his daughter and son in law and says 'Well guys, welcome to family life'. Everyone laughs loudly at this because he is a ruthless king.