What a time to break down! The TARDIS is old, was an obsolete model when he stole her, but the Doctor’s never coveted the newer models. She’s his friend, his travelling companion, and his home, but like anything with mechanical components, a certain amount of wear and tear is inevitable.
It’s fortunate there was a nearby planet he could set down on, but bad luck that this world is ninety percent oceans and they missed the nearest landmass. Now they’re all at sea. Literally.
Perhaps if he sends out a distress call someone will come and tow them to shore.