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There are no Abyss Diving Spots, only Abyssal Underwater Structures.
You can find Abyssal Underwater Structures with luck when looking underwater, using a sea chart, or having a diver title (in their respective sea)
Find them like you would with normal ones, e.g. sea charts or having nimbus diver badge. The ones with purple depth numbers are deep enough to get the badge.
An example of an Abyssal Structure. This is on the deeper end of what I’ve found, but you’ll notice that the usual reds shift slightly into purple with most. 170-ish seems to be around where they start (might be shallower).
how much air do you need?
Quite a lot.
I don’t think default leaves you with enough to get back. Iirc, waterbreathing III still gave me a bit of trouble, but is doable.
I’d recommend either high tier water breathing or just get 5 water breathing musgravites and negate breathing entirely.