Sure, Seamus resembled a scarecrow, but it wasn't like he was going to ADMIT to it now. His ego was fairly large and it would be a struggle to deflate it or allow Dean to kill it with his vicious, vicious lies about his complexion and (lack of) muscles. He would rather have something blown up in his face than let that happen.
Seamus, unlike most male teenagers, really did like playing with kids. Being an only child with only grandparents as playmates when he was younger had definitely left him wanting and when Dean and his billions of siblings had fallen into his lap? He had jumped at the chance to immerse himself in make believe games and tea parties. Dresses and all.
"OY!" Seamus bellowed as he was sprayed with freezing water. He would have thrown up a shield, but that would have sent the water splashing everywhere and he didn't want to break anything. So he hunched his shoulder over to protect himself a little, before walking swiftly in the direction of the stream of water and Dean. His wand had been tossed to the safety of the table, so both of his hands were free to try and pull the hose away from Dean.
Who was both bigger and stronger than him. It didn't help that Seamus was completely soaked.