THe Corps has a page with Smithville Lake conditions at:
http://www.nwk.usace.army.mil/cgi-bin/daily.pl?site_name=smithville
Today, Sunday, May 18th, it's showing surface water temp of 62 degrees. That's rising nicely - and in another few degrees it'll be warm enough for almost everyone. Right now some would still call it pretty chilly but it's warmer than the California water gets and swimable if you jump in, splash and make a lot of whooping noises as you thrash you're way into deeper water.
Dick