We were there in mid-October. The water was probably in the 60's (like the Pacific in California in the Summer). The kids swam some, we just waded. The kids climbed down a ladder to get in at Vernazza. We all waded in at Monterosso.
CT isn't really a beach place, but it's nice to wade in a little when you are up in Monterosso.
I always pack either a swimsuit with pockets that can work for shorts or a pair of non-cotton shorts that can double as a swimsuit.
Unless we are going on a beach vacation, my wife packs one one-piece bathing suit that can also be used as a body suit with a skirt and dressed up with a sweater. On beach or cruise vacations, she brings at least two one and two-piece suits.
If CT is your only water location, one suit is plenty.