From what I've read this may not be the best strategy. I considered it too, but have since discounted it.
Most people who have already taken the test and have written here state that the morning section is easier than the afternoon, in terms of problem difficulty. Even though you may know more about construction, you might find the afternoon questions very difficult, whereas those 8 morning water questions were easier to get.
I keep reading a reasonable strategy is to go for at least 35/40 morning questions, at which point you only need 20/40 afternoon.
Also, why water? In my mind some of the concepts tie in closely to the geotechnical subject that learning both reinforces each other somewhat. I'm taking transportation, and if I was to drop one from my study schedule I'd drop structures - which doesn't show up on my afternoon portion and is therefore only 8 questions total. But I've decided to devote time to it hoping those 8 questions will be easy enough for me to answer.
If you need to save time, a better strategy is for water, don't go in depth with your studies on water or anything else you don't make the time for. If you learn all of the CERM water chapters it is likely overkill. If you instead get comfortable with Goswami's easier textbook than maybe that'll be all you need for the easier morning questions. Or if you stick with the CERM, probably most of the questions will come from chapters 17, 19 & 20 - just 3 important ones to review really.
