@luxiaojun177 They were expecting opposition; page 10-11, the officer commanding the relief effort repeatedly issues commands to their attending warships. You don't bring warships unless you're expecting to fight - until the modern day, they are generally so precisely constructed that they have no other useful purpose.
The plan wasn't for them to slip by unmolested - the plan was for all the land battles to draw so much attention away that the river was lightly guarded and they could smash through it with little trouble. Obviously, Riboku saw through that and mustered what is probably a significant majority of the Zhao fleet.
@defianze If they come at all, it's going to take a while. Chapter 574-575; Shunsuiju is still guarding them with a sizable army, plus all the extra bits and pieces Zhao has to throw around.