Capillary action, a nightmare in a case like this, water has got into every single joint and done its thing. This is a bad case especially on the tommy bar where the wood has split right open, and the top boards look to have delaminated as well, and the dovetails on the tail vice are popped out of joint...was it in fair condition before it got wet?
A lot depends on what exactly you want to use the bench for, and how confident you are at taking on joinery?
A purist cabinet maker will aspire to a perfectly smooth, flat, perfectly level "refererence surface", for doing very precise measuring, setting out, fitting, test assembly adjustment and final assembly. That is the top level accuracy, which most of us have no real need for. On the other hand if you are doing less demanding work, you might get away with levelling the top after letting the wood dry out slowly, or simply fasten a sheet of 18mm ply over it? Personally, seeing as the sub frame is sound, I would consider removing the top and vices, and build a new top, maybe get a record vice off ebay, leave the old top to one side until such time as you are confident or inclined enough to rebuild it...