Take all the varnish off and start again. I would use epoxy to fill the cracks, but lots wouldnt. Might need something more flexible.
But really all that varnish needs to come off, the wood has bleached also, I think it's Mahogany, so will come up a lovely reddish colour, once all the varnish is off and it's sanded. The whole boat probaly needs re-varnishing, if the varnish is all that old.