Thanks to +Nobilis Reed for the question.
The answer is: It depends.
Generally, horses like water as long as it's safe. Horse owners might, on a hot day, put a small wading pool in their horse's pasture, and the horse will play in it and enjoy it. Riding into the sea on a sloping beach is an equine vacation staple (and working horses in salt water is sometimes practiced by racehorse trainers as it can help keep a fragile animal sound).
However, horses will not go into water if they can't see the bottom of it. This is sometimes done deliberately on higher level cross country courses - the point is that the horse should be willing to believe and trust the rider when they say the water is safe. It can be very hard to convince a horse to go through even a familiar stream if it's swollen and full of mud. In this case, the horse is worried about getting stuck in the water (horses also have a very high instinct for whether quicksand or similar is present).
Another thing that will put a horse off entering water is thick overhanging branches or vegetation, which might conceal a lurking predator.
Incidentally, sometimes when a horse plays in water it might not be that much fun for the rider. Their idea of "too cold" is not our idea of too cold and they will cheerfully splash you thinking you'll like it. I've also had horses attempt to lie down and roll in water, which can ruin their saddle.
This feral horse is rolling in the ocean in North Carolina, apparently for the sheer fun of it (or to cool off). Image source: Kersti Nebelsiek via Wikimedia Commons.