So I’ve been planning a Disney trip for May that would have included Typhoon Lagoon, which would be our first time there (we did go to WDW last year but only went to Magic Kingdom and EPCOT). Due to a changing work schedule, we might move the trip forward to March 15th. What’s the weather that time of year? Will it be warm enough to go to a water park? Even if the water is heated, will we freeze as soon as we get out?
You will be fine. I've done Disney water parks in January and my DH and DD went two weeks ago and they were fine. On the occasional chilly day in FL the water parks won't open, but it is very unlikely to be that cool in March. If anything try to have some flexibility in your schedule and go on the warmest day of your trip.
So, in 1990? I think? (HA!) we went to Disney first week in March. Typhoon Lagoon was new but we did go there one day. I think you could find a day to do it unless you are in a cold spell.
I went with my kids first week of March 2019 and the weather was warm enough for a water park but not necessarily every day.
I would think by March 15 you would be in decent shape, but of course you could always hit a cold week.
Google says it's not opening until March 17 this year, so that might impact your decision depending on how long you'll be there. FWIW the closest to your dates I've been at Disney was Leap Year Week ~8 years ago, and it was way too cold to think about swimming. The hotel pools weren't even open because daily highs were in the low 60's.