Taking a boat trip is great; you can listen to the commentary, enjoy the scenery along the banks, and even hear about some attractions you might have missed. I recommend taking a boat trip on the first day, as it will help you plan your itinerary for the rest of the trip.
For example, there's no need to go to Merlion Park; it's crowded and sunny, and you can take photos from the boat 😄. Alternatively, you can choose to go in the evening when it's less hot.