I'd go with a full drysuit. You can have long days on the water, and many sections are in shade. The water is always cold, and if you have separate pants and top, you will get leakage and be soaked by the end of the day. If its hot during the day you can splash off or swim to cool down. I went down the Grand at the end of september, a similar somewhat shoulder season as April, I would think. There were a few times when everone in the water (kayakers) was really cold at the end of a long day in the shade.
If you have a paddle top and pants etc, I'd bring that too. Having a backup set of gear on a long trip is not a bad idea in case gaskets blow, gear gets damaged etc.