How do I resolve issues with my translation of a snippet about 观潮 (guāncháo = tide watching) from a 4-th grade Chinese book?
海塘大堤 is a particular type of levee. Not all levees are seawalls. See this and this.

横卧：It as a figurative use. The river is there like a person lying there. Something like that.

越往东越宽: There is no verb here. So no "moving".
往 is not a verb here. It is a preposition that means "towards".
This just describes the rive, which is wider in the eastward. "The more eastward the wider".

昂首东望: 望 is not "hope" here. It means "look". The phrase means: raising one's head and looking east.