We do a lot of freshwater swimming up here -- it's always on the cold side, even in the summer -- and I'm like you, I like to just jump right in. It's a bit of a shock but I get used to the water temperature more quickly that way. Re: our March visits to CC, it was sunny and in the low 80s both times (I checked our old Navigators to make sure I wasn't misremembering). For us, that's balmy.
Low 80s and sunny is beach weather!