When an American visits Australia, how noticeable is their accent to yall ?

Well, probably the most overt feature is the rhoticity - https://en.wikipedia.org/wiki/Rhoticity_in_English

That's a bit like asking what the colour red looks like to others, but we will very quickly recognise a US accent. We have US TV, US movies, US youtubers, US news, just constantly around us all the time. We're very familiar with the accents.

I know accents pretty well. I picked up a US accent living there as a child and worked extremely hard to sound Australian again when we moved back to Australia, and I've also done a lot of acting involving accents. You can learn about various accents and listen to samples at the international dialects of English archive: https://www.dialectsarchive.com/

That doesn't tell you much about what Aussies will think, but it is interesting, especially if you want to learn how we might pronounce things.

Most Aussies wouldn't know it consciously, but on an unconscious level they'd recognise New York (George Constanza, HEY I'M WALKIN' HERE), Bostonian (Bill Burr, Good Will Hunting), Appalachian (Forest Gump, Deliverance), Texan/Kentucky (Justified, those nice boys on Supernatural, cowboys in general), West Coast (surfer dudebros), midwest (Fargo), and received pronunciation (news readers, college educated in general), AAVE (Chris Tucker, Chris Rock, Samuel L. Jackson), and probably a few others I can't remember off the top of my head.

Generally visitors will say that people here are really friendly to them. Public transit is good, especially in cities and places tourists go. Sydney is the most beautiful harbour in the world (no bias here of course). Anywhere you go or eat within shouting distance of the harbour bridge is going to be nice, if a bit pricey, and I would recommend just about anywhere on the harbour that you can reach by ferry. If you go past the heads you deal with sea swell so you'll want sea legs for that.

Exactly the same as it sounds to you. It's just that we don't think our accent is unusual and you don't think yours is unusual.

No we don't say y'all. Often it's 'Youse'

Just repeating :)

No worries...

Now, lets talk about giant spiders and deadly snakes, how do you feel about those?