Is their feed soaked because it's grain? If so it will absorb a lot of water. If not, why are you soaking it? Are they getting lots of waste from the vegetable garden? This will also contain a fair amount of water. Pigs drink a lot in hot weather if they're active, or if they're lactating, but otherwise often only after a feed. How fresh is the water? I adhere to the maxim that you only offer water you'd be happy to drink yourself. Something full of dead flies, or that has absorbed the taste of the container it's presented in, or has gone green in the sunlight, is not going to appeal to a pig.