In the swing season, March-April, it can snow into the Redwall with a cold rain at the river, and in another couple of days be shorts and t-shirts weather at lunch.
For some folks, dry suits can be hard to get into and out of, claustrophobic and too hot in parts of the day.
Another option is to bring a good rain jacket with a hood and removable liner, and sailing bibs with ankle gaskets or velcro closures, like what Carvedog was saying. The rain jacket top is easily removable when it warms up, and this arrangement allows for lots of layers. Having taken a few... um... unintentional swims in this set up, i have found water seeps in, but doesn't exchange fast, so while i was wet, i was warm and could swim fine and get back to the task at hand once back on shore. Then change into dry clothes. So you have options. In the low-rider GEM boat on cold big water days, I wear a velcro waist pants bottom with integral booties, and a spray top with a fleece cap and sandals, layered inside with fleece. On the cold not so big water days, am back in the bibs and irrigation boots from Cal Ranch Supply. Another key part of this outfit is a high poundage flotation PFD. Air is a good thing, and having a life jacket that can get you back to air-land no matter what you are wearing is a good thing. Have a GREAT trip, yours, tom