Is content on hover only a WCAG failure?

I'm looking for some conformational feedback on a simple question: If content appears only on hover, i.e. the trigger is not focusable, does that *always* constitute a WCAG 2.0 failure?  In the context of 2.1.1, I suppose this asks whether the ability to show the content is considered "functionality" and thus must be keyboard operable.  One example might be where a help icon is used to show extra info for a form field and is piped to the accessible description:
<label>Reason<input type="text" ... aria-describedby="tooltip"></label><img src="help-icon.png" ... onmouseover="showTooltip()">

Depending on your answer above...

If you answered yes, then doesn't that translate to saying that any element with a title attribute must be focusable, either by default from its semantics or manually via setting its tab index?  For example:
<img ... alt="blah" title="more blah" tabindex="0">

If you answered no, then can you offer examples and explain why they are fully accessible (I don't think mine above actually is)?

Steve Repsher
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