It's often hit or miss.
We have been several times in April and May and it has been too cool to swim. In fact, our last trip was in mid August and while it was perfect for swimming, we needed a light jacket to go see the fireworks at night as it got quite chilly.
Having said that, we took a trip to DL in early February a couple years back and it was close to 90 degrees everyday and we were able to swim without any problem (that's definitely not usual for Anaheim).
You may get lucky and get some nice hot temps for March, so I would bring those bathing suits along, but get your mindset ready for the fact that it may just end up being too cold.
You will most certainly need sweatshirts or jackets for nighttime activities.