Beach/swimming dialogue. 浅い 'shallow' describes the water. 足とどく (= 足が届く) 'feet can reach' is a common expression for being able to touch the bottom while standing in water, with the subject 足 implied and particle dropped. 遠浅 'shoal, gradual shallow' is a specific word for a beach where the shallows extend far out. だろ is the casual clipped form of だろう. Two characters (one adult speaker, and reference to a child Yotsuba, a common manga/anime character name) discussing safe swimming depth.