I would recommend taking him through some shallow water ( maby knee height ) to start with following another horse that is experienced and has no problem with water.
Puddles are almost slightly different ....the way I see it. For instance , my horse Banjo has times when he will have a little shy at a puddle...but he looooooves water other wise.....if its say knee height or more. He's been swimming before with me, and heck we have even galloped through some water , not too deep tho....just a bit below the knee.
So I would ...yea ....say find some place with a streach or section of water....more than that of a puddle , and follow some one experienced through. Be aware about the horse wanting to roll too, let the horse touch the water and investigate but be ready to push them forward at the first sign of wanting to roll.
I did this when taking my horse foxy out. He was a show horse and seemed to have never been ridden outside of paddocks/arenas. Took him through water at the beach which was at his knees or a bit higher following my other horse. He was a little nervy at first but followed. I let him have a snif and lick...basicly just let him touch it and investigate. He's not 100% about it yet but enough to get through saftely. And he doesnt mind having a splash