I've been at the end of January and the middle of Febuary. It really depends on what the weather is doing in general and if the sun is out.
I believe its the lazy river and the normal pool that are heated where they'd actual feel "warm". The animal pools are kept at a temperature that's correct for them, which can feel chilly to humans. Honestly once you're in the water for a little bit you get used to it. They have large heaters around but I didn't find them necessary once the sun came out. The wetsuits dry pretty quickly too so its not like you're soaking wet all day, just a bit damp.
The wetsuits come in a few different styles and you MUST wear one while in the water, at least with the animals (I don't recall if I saw anyone in the pool without one) They have a colored strip on them that basically identifies you as a guest, and the trainers suits have a different color. There's a vest which is the easiest to get in and out of but will offer you no warmth really... there's a 'shorty' version which would cut off at about your knees and elbows and has been the option I've always gone with. Its tolerable to get in and out of and has been enough for me for warmth. There's also a full length option and I've seen a few people wear them but I can't fathom getting in and out of it.
When I was there in January, it was a bit overcast in the morning and quite cold. Once the sun came out it was much better, but the breeze plus wet wetsuits got a bit cold, so it was just easier to stay in the water. You could also try to request a late morning/mid afternoon dolphin swim time so that the sun would be out and have a chance to warm things up a bit and do the aviary and heated pools until it does warm up.