No attunement required on this?

Nope

Looks like this doesn't work with Tritons on ddb because of their innate swim speed. 

Does this allow someone with armor on to swim 40?

I might be wrong, but I am pretty sure there is no rule that says armor effects swimming. Swimming is just half movement. The ring just make it so you can go faster in water plus some other stuff tied with swim speed like attacking underwater.

Would this ring grant someone the *ability* to swim? I have a PC who can't swim

In this case, the DM could rule both ways. If I was the DM I would say yes, since its a magical ring. But the character may have a lack of faith in magic too, and although they get the ability to swim, they may THINK they still can't swim, which could be a funny roleplay situation lol

It's completely up to you, but I would say yes

I really dislike all these cheap items that give swim speeds that are faster than what playable aquatic/amphibious races get (30'). It feels like it makes those racial traits nearly obsolete because any time you are traveling underwater with your party you'll be quickly outpaced unless you also use the same magic. Except for that one time during the whole campaign that you need someone to swim down to grab a shiny in a lake.

Personally I think anything less than "very rare" should be nerfed down to 30' swim speed.

The broom of flying is only uncommon, man just get over it, swim speeds are situational and a weak feature generally. They don’t want players to be useless in the aquatic sections so they make it easy to get, that’s why water breathing is a 24 hour ritual at only 3rd level. You can just ban the item if you feel that strongly.

Well the conceit of 5e is that magic items are nonexistence basically. And everything assumes they don't exist.(I think 5.5e is different.) So I think that when they made aquatic races they just keep the 30' as standard and didn't even check the items. 

Make an underwater thing with an anti-magic field or whatever then they can get the buff. I think if you make the item require attunement it would be fine.