Approx 30 feet of 8/3NM cable and a two-pole 40 amps breaker to fit your panel.

Thanks - that's correct - tandem breakers. Can you post that link? here's what I meant: http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100021761 cheers

You definitely can't just randomly replace the 80A breaker with a larger one. Even if you did, you'd be limited by the 70A breaker outside... Don't even think about replacing that. 60A (what the water heater needs) is less than 70A (what you have), but you'd end up with some situation where you can use your hot water ONLY if you turn off everything else in your house. Do you want that? Basically, forget it. You'd need a service upgrade for this to ever work in a reasonable way.

Actually, the 2017 code did indeed change this. I stand somewhat corrected. In 2014 and earlier codes there was an exception for dedicated refrigeration circuit. In the 2017 code, it was changed to any single receptacle for "a specific appliance." This circuit could be 15A. If you have a circuit with receptacles for multiple appliances (say a disposal and a dishwasher) it needs to be 20A. As for GFCI, the operative change for dishwashers was in the 2014 code. Prior codes said "within 6' of a sink in other than kitchens". The "other than kitchens" was removed so that undersink receptacle in the kitchen needs a GFCI. Yes, a GFCI needs to be installed in a "readily accessible" location. Now the question is whether pulling the dishwasher to get to it is contradictory to "readily accessible." Article 1 pretty much says that means you don't have to use tools or remove obstacles to get to it. The dishwasher is a pretty big obstacle. I've always put the receptacles for the dishwasher in the sink cabinet anyhow, even when it wasn't required by the code, it's a royal pain to do otherwise.