Short answer: Marazion itself has a free museum, three good galleries and a couple of pubs with a view of the weather — an hour or two, not a day. For a full wet day, Trengwainton or Godolphin (both National Trust) or Penzance's indoor options are the better call.
Wind and big seas can close the Mount at short notice, and the beach is no fun sideways. Here's what Marazion can honestly offer when the forecast goes wrong, and when to admit it and drive somewhere else.
National Trust - Trengwainton Garden on the far side of Penzance is a garden, but a sheltered one with a café and enough tree cover to make a wet walk pleasant (closed Fridays and Saturdays). National Trust - Godolphin five miles east has the house itself, a barn with a film the children like, and a café. Both are an easy hour and a half.
If it's properly wet all day, Penzance has the indoor options Marazion doesn't: the Penlee House gallery, the cinema, a full high street, and the Jubilee Pool's heated section if you're the swimming-in-rain type. It's ten minutes by bus. Our Penzance guide has the details.
The Mount announces closures on its website and social media and emails ticket holders; if nothing's announced, it's open. Drizzle doesn't close it — wind and swell do. A grey, still day on the island is quieter and rather atmospheric.
See the full Marazion guide, filtered by who's with you.
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