From early in the year, Shark Boy had been swimming. Visits to various lakes and pools had only strengthened his ability, and made him more Shark-like. The arrival of summer meant regular visits to the Castle of Summer. Seeing Shark Boy enjoy the water fully immersed brought joy to the King’s heart since the image was what he had always pictured for his family to enjoy being lakeside.
The Lightning Kid enjoyed the water too, though maybe not quite as much as his older brother; he didn’t seem very willing to be carried into the deeper, colder water, but he was game for wading under his own terms. Unfortunately, those terms sometimes meant tripping or being knocked over by a wave, which was a problem.
You might recall that the Lightning Kid had Tiny Titans placed in his ears, and their presence meant water had to be kept out (lest they drown!). Lake water was especially suspect, since it could more easily lead to problems in the ear. What the King and Queen didn’t know, is that the Titan of the right ear had long since departed, and the ear had become infected. The Lightning Kid’s behaviour deteriorated in the middle of the summer, but without a fever to indicate anything, the ear infection went undetected for too long.
When a fever did appear, they took him to a healer who found that the eardrum had been perforated. With medicine, the infection was cured, and the eardrum healed fairly quickly, but they were more determined than ever to keep water out of the ears.
There were ear plugs that could be used, but they would either fall out on their own, or be removed by the Lightning Kid. A better suggestion proved to be a magic hat that looked like a piranha. The Lightning Kid was not happy about wearing it at first (especially because it squashed his face)...
... but soon enough he associated it with water fun, and the King learned to put it on so that his face was less squashed, and it could be used to hold the earplugs in place (by covering his ears).
Still, the King sought to bring the Lightning Kid along on the aquatic adventures he could share with Shark Boy. If he were to carry him into deeper water, the King did not feel fully confident of his ability to keep the prince's ears out of the water while swimming, even if he could carry him well enough to keep him generally safe. They set out to tame and conquer a crocodile that the little prince could ride and keep his head well above water.
Once mounted, the Lightning Kid did a wonderful job of staying seated, holding on, and keeping his balance. So much so, that the Lightning Kid, Shark Boy and the King were able to conquer a floating island.