Wooden Side Decks

Nothing on the underside at all? If that's the case you need to remedy that as a matter of urgency, but hard to believe anyone would be that silly. What's the history?

sounds as if it might be good to get the boat under cover for a couple of weeks to dry out thoroughly then revarnish and do the undersides if accessible.

I'm a very long way short of the worlds greatest wielder of a varnish brush, but what exactly is the problem. Is the varnish coat actually compromised? If the decks are unvarnished underneath I suppose its possible that damp has actually helped lift the varnish from the wood... I personally aren't convinced its necessary to go back to bare wood as often as folk do, but you do need to get the moisture content to something healthy and get an unbroken cover over all the wood.

Sand it back hard until it is smooth, then revarnish. Agree you don't necessarily have to strip it back, unless areas are lifting. But before that turn the boat over and redo the undersides of the deck. The tops will never look as new, and once the cracks start, they have a habit of coming back. They seem to whether you use epoxy, G4 (or other cyanide based primers) or more normal varnishes. However, plenty of coats and keep an eye on it and you should be fine.