I would say pre-order it for 100% sure! We turn our pool heat on still in May!! The pool heaters work via a heat exchanger...so once you arrive, even if you decide to turn it on...and your MC or owner comes out that very same day and turns it on, it's going to take a day to get warm. Now...if it's Sat afternoon or Sunday when you decide it's too cold and want it turned on....they may not come out until the next day...which means you will go several days without it being turned on, for instance.
We have had several guests who didn't opt for pool heat in October or other cooler months, almost universally they all commented afterwards they had chosen to have the pool heated.
The one other poster's comments about the "pools being heated" refers to the resort or water parks pools, not private vacation homes, where pool heat is almost always optional.
In April, I can't recommend this to you strongly enough!!
You're almost certain to get there and find it too cold for your liking, then wish you had pre-ordered it so it's nice and warm when you arrived! The very first thing our kids do when we arrive in Florida is jump in the pool, it's one of the highlights of their trip, sad to say, but they almost enjoy it as much as WDW! LOL