Why is he so aggressive all of a sudden?

how does he attack?
like "serious biting, blood and shaking"-kind of attack or "I grumble at other dogs or snap in the air because I feel ****ty and don't want to be bothered by some obnoxious puppy"-kind of attack.
in first case you definitvely should be keeping your dog seperated from other dogs and need a behaviourist/ trainer that's experienced in working with interspecies aggression, if it's the second, keep dogs away temporarily until he feels better and then try again.
either way muzzle training is never a bad idea, with any kind of dog.

Phone your vet and tell him about this. It could be a symptom of whatever made him violently ill, he may need more meds. In the meantime, protect him from interactions with dogs, people, anything that might upset him. If this ONLY happened immediately after the 'water poisoning' (or whatever it was), it could be just that he is feeling terrible and therefore protecting his space untill he feels less vulnerable.
I once had a serious knee infection--more than the pain, I had the distinct need to have a 10' safety zone around me. I wanted no one, not even my husband in that zone. It was a very instinctive feeling. And I was seriously cranky without a good reason--and it wasn't the pain (it wasn't that painful unless I moved).
So, phone the vet-- could be a very important piece of information.

Its resolved. He was still really unwell. I had to bring him back down to the vet only a few hours after writing this post because he was vomiting again. They kept him in for 3 days on a drip and different meds. They took bloods and an x-ray and everything came back fine.
He has been in great form since and hasn't been snappy at all.

Poor doggie was just sick. Thank god he is better now, I was so worried all weekend.

Yay, thanks for the update.
I thought so.
Yes, some types of pain, discomfort, can be so bad that even the most normal peaceful person or dog will snap.
Glad things have turned about.
What in heck was it? Do you know? Blue-green algae poisoning? Some waterborn bacteria? Just don't know but it's better now?

my vet didn't know exactly what kind of bacteria/algae was to blame for it but he did say that he had a number of other dogs in with similar symptoms, however my fella was much worse for some reason.
Thankfully he is still fine, in great form, playing and full of energy and eating well. So hopefully that's it all over.

I was super worried because a friend of mine had a full grown old English sheepdog who died of similar symptoms and he contracted something from the water in the same dog park - about 4 years ago and I was terrified I was going to lose my dog at one stage, he was so ill. Its not even worth thinking about, I have cried more in the last week with worry than I have in the last year:eyeroll:

Thanks for the replies