The lake is open year around. The closed signs that you are talking about might be at where the river leaves the lake and at the head where the inlet comes in(Little Wenatchee River). But all salmon must be released,including all the Kokes. Found this info on page 128 of the regs.

Just checked on it's size and it is 2445 acres. I don't think a float tube is big enough. You could get lost in the waves if the wind ever came up.