Hiccup's prosthetic wasn't perfect by any means. If he forgot to take it off at night the base of his leg would swell up and he could forget about walking on his own the next day. It was heavier than his actual foot and therefore put him off-balance and made him twice as clumsy as he had been before. Should Hiccup move in just the wrong way it would shift into an awkward angle and he would need to fight with the prosthetic to get it off an put it back on; a problem that occurred usually when running. Sometimes the straps were too tight and would cut off circulation to the bottom of his leg. When on the shore, the prosthetic would sink in the sand and the tiny particles would get in the junction between his real leg and the extension. If he wanted to get in the water or take a bath or had to leave the house while it was raining he had to either take the prosthetic off or stop to dry it every half hour to keep it from rusting.
It was too heavy for him to swim in, ans it's shape made it impossible to catch water to help propel him to the surface.
As Hiccup sank further, his false leg acting as an anchor, he contemplated the changes he would need to make in order to avoid this in the future. Should he make it, that is. Hiccup was always more of a realist than an optimist. He knew he was too far down to save himself, and the battle on the surface was still raging on if the occasional weapons being thrust under the water with him were any indication.
At first he had been able to keep his head above water, but with just one working leg and his arms he grew tired quickly, the on-going attack too loud for anyone to hear his cries for help. He drew in as much air as his small body would allow before he was drawn under by the weight of his prosthetic.
Now he just continued sinking, un-moving, only occasionally letting out small air bubbles as he tried to delay what now seemed to be his fate. Hiccup began to fear he was too far down for anyone to even see him, the light above the surface hardly reaching his eyes, and then he knew he was too far down for anyone to reach him.
Minutes later, Alvin and his men retreated, leaving Stoick and the other men of Berk as the victors, but it was bitter sweet. There were several injuries, and a few dead, but looking to the sea their enemies had suffered a much greater loss. This was only supposed to be a fishing trip for a group of Berk's men, a time of laughter and story telling and mead, a time to bond with his boy…his boy! Stoick stormed the ship, hollering at the top of his lungs for his son. He knocked over barrels and crates, shoved his injured companions who got in his way, and nearly pushed Gobber over when he prevented him from jumping into the sea in search of his son.
Gobber had never seen Stoick cry before, and should anyone ask him he would say he never had.