March is EASILY a good swimming month, IMO. Heck, my best visits to Typhoon Lagoon and Blizzard Beach have been in January! (depending on which one was not being rehabbed at that time in a given year). Granted, many days in January are too "cold" for most people for waterparks, but there is always days where the high hits 70 (or above), and combined with the sun, those days are more than hot enough to visit those parks for me.
Since school is back in, combined with the fact that it's usually not a MISERABLY hot day (80 degrees or higher for me), most people at WDW in January are not really in water park mode, and you can have the park virtually to yourself, especially on a weekday! I've heard the horror stories about these waterparks reaching capacity during spring break and summer, and the long lines, and I know how miserable a waterpark can be on a blistering hot day where your bare feet feel painfully hot just from walking around.
But on a nice January day in the low 70's/upper 60's, none of this happens, and being able to ride the slides over and over again with NO WAIT (or minimal wait) to your heart's content is truly priceless.
So to answer the OPs original question - for me, I would say the VAST majority of March days are swimmable days, and even many evenings. By March 1, the average high at WDW is already a warm 75 degrees, and by the end of the month, a stuffy 80 degrees. As for how many days in the month are not good swim days, will depend on if the month is cooler than usual, and will very by person and what their threshold is for what they consider "too cold".
http://www.weather.com/outlook/events/weddings/wxclimatology/daily/USFL0615?climoMonth=3