Yeah, they can weigh up to 2000lbs with a king-size bed. A king-size bed is 6,080 sq in.

A fridge can weigh 300lb being 36"×30". 1,152 sq in.

Fridge is .26 pounds per sq inch. A water bed is .33 pounds per sq inch.

So while heavy the weight is distributed basically like a fridge. This is assuming an empty fridge.

As for durability, a quality waterbed mattress is thick. You aren't going to pop it or cut it without deliberately trying to.

Even if you took a knife and stabbed it from the top, it's not going to leak until you put weight on it.