My general rule of thumb is that I kayak when the sum of water temperature plus air temperature is greater than 120. The risk of hypothermia is much less when the water temperature is above 55 and the air temperature is above 65. This usually occurs sometime in May and lasts until October. Today's water temperature was 65 and the air temperature was 75F with a very light wind - perfect conditions.
The route I take passes through the "Lakes District" of the Charles - a wide, shallow, and particularly beautiful stretch for flatwater paddling.
I generally kayak between 5:30-6:30pm, stopping at the Charles River Canoe and Kayak boathouse on my way home. It's a great way to get some exercise and decompress - I can work a full day, kayak, have dinner with my family, and the continue to read and write until bedtime.
The early evening on the Charles is a perfect time to view widelife - Great Blue Herons, Trumpeter Swans, American River Otters, Snapping Turtles, and enormous Carp. The river is different every time I kayak with variable weather, changing wildlife, and new people. Today I did a rescue of a mother and daughter from Montreal who tipped their kayak in the deepest part of Charles. We did a T-rescue and all is well.
The boat I prefer is the Epic Kayaks V10 Surf Ski, a fast 17" wide boat that's a racing and fitness kayak, just unstable enough to keep the trip very challenging. In windy or turbulent conditions, I'll pick the Epic 18X. Here's my guide to choosing a kayak.