First of all, if he's thirsty enough he'll take the risk to drink. Dogs will act on instinct like any other animal when the time is called for. Why do you think African animals drink from water holes that could have crocidiles in them? Because they need to, to live. He'll learn that this isn't that scary once he realizes just how thirsty he is. Second of all, if you are that worried about him, get another dish for the time being and put some water in it just so he doesn't get too thirsty.

He's already drank from it, so he knows he can get water from there. When he's thirsty, he'll drink from it again. [edit] If you make a big deal over every little thing, then he'll learn all he has to do is act a certain way and he'll get anything he wants, just like with a child. Do not give him another bowl, let him learn to drink out of what it provided for him.

Get some really prime treats...such as boiling up some chicken livers (which are really cheap to buy). Cut them up into very small bits. Take SOME of the bits and plop yourself by the water dispenser...on the floor with the puppy. Encourage him to come close and feed him the bits of meat resting your arm ON the water dispenser...your goal here is to make the water dispenser part of a normal MOST EXCELLENT AND REWARDING TREAT. If you have to...once he is coming near the thing, set a whole liver IN the drinking tray so that it pokes up and he is encouraged to grab it. It doesn't matter that you will have to clean the tray, nor if he grabs it and jumps back to eat it...the point is he put his snout where it needs to be to drink AND IMMEDIATELY GOT AN INSTANT REWARD for doing so. Also, make that a play area, as he becomes more used to it..MAKE IT GURGLE...don't say, "oh poor puppy, it's ok..come here!" Instead laugh like hell, pat the jug, laugh some more and keep playing, assume the play position..on your knees, butt up in the air, head down...show him this space is a ripping good time, and NOTHING to fear. Assuming the play position is DOG BODY LANGUAGE that he can understand. A scared dog does NOT assume the play position..roll over on your back next to the thing and wiggle...same difference, a scared dog might lay on it's back in submission, but they won't roll around, toss a toy and generally appear to be having a very silly good time. KEEP THE TREATS COMING during the play time. If he looks at you as if you've cracked your head, you're doing it RIGHT...you are making him think and wonder, "maybe this thing isn't so bad, Mom/Dad's having a good time, doesn't seem worried about it...hmmm, I LIKE these treats too! Any more in the bowl?"

I got a 3L contraption like that for my cat and with the current heat wave I'm glad I did. However, a 3 gallon unit would be much bigger than mine. Regardless, the method I used to get her to drink from it (she was scared by the gurgle as well) was to take away all other water sources in the house, then constantly encourage her to drink from it. Spend time near it, dip my finger in the water and hold it to her nose (so she can smell what's in there and realize it's just water), and letting her experience it on her own. After about 12 hours without drinking anything (this was before the heat wave) she finally decided to try it. I turned everything off and made sure she was alone, watched from a corner, and she finally started using it. Now it's a regular part of her day. Might work a bit different for a dog but you never know.