Hmm... good question. My dog doesn't like water either, but she's forgiving enough that for most of her life I've just... held her in the tub and talked to her in a calm voice and gotten done as fast as I can. Then she slides around the carpet on the side of her face after she's shaken off and been toweled a bit, and continues to rub her entire body on carpeting or grass until she's mostly dry. Sometimes I brush her out in the meantime. Obviously, that doesn't help you much.
I guess I would try first to get him to stand quietly in the tub, without water. Stand in tub, then be calm in tub, click/marker word/whatever and treat. Then I'd let the water be running gently but not touching him, again, click/treat. Until he's completely cool with that. Then just enough water to cover his toes. Same routine. On up until he'll let you put water on his body and actually wash him. Don't hold him down in the tub, though if he'll let you gently hold him that should be fine I'd think. Speak to him calmly and don't treat it like it's a big scary undertaking.
No clue if that will actually work, as I've never tried it, but that is what I would try if I were going to attempt to teach my dog to be more okay with water. I've taught her to be willing to wade in quiet water by similar means...