、、。
It came up out of the water and started toddling unsteadily over the sand at the edge of the inlet.
Literal
Water [from-から] ascend, inlet [genitive-の] edge [genitive-の] sand [genitive-の] top [along-を], totteringly began-to-walk.
Picture-book or children's-story narrative about some small creature (likely a turtle, penguin chick, or similar). よちよち is a 擬態語 for the unsteady, toddling gait of a small child or animal — the hallmark Japanese word for 'toddling.' 歩き出す is a compound verb: 歩く ('walk') + 出す ('begin'), meaning 'begin walking,' 'set off walking.' The particle on 砂の上 is を, not で or に — the 'path-を' use, which marks a route or surface that an agent traverses: 道を歩く ('walk along the road'), 空を飛ぶ ('fly through the sky'). Here, 砂の上を walked-along = 'walked along on top of the sand.'