I'd just wrap a towel around me going between the slides. When queuing in the towers, I can put up with being a little cold sometimes if it means having a heap of re-rides & not being lucky to get one or two goes on each. Most of the time it's not going to be THAT bad. You can always wear a rashie or t-shirt to help stay warmer too.
But back to the lazy river -- I don't mind if it's something that isn't run all year but if it's heated, then they could always rotate staff between other slides to compensate. Once you're in the water, you're used to the temperature anyway... and if you get cold floating on top, just get in the middle of the tube and kick yourself around...