My take:
A performance boat is any boat that's fast or highly powered. It could be a 100mph bass boat, a drag boat, hydroplane, river rat, etc., etc.
An offshore boat is a special class of performance boat which by way of it structural integrity, hullform, weight, etc., is capable of high-performance operation in rough seas as would typically be encountered offshore - something one shouldn't consider in other high-performance vessels.
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