Below Bonniville, when I am fishing gators, I carry 450' of rope and 8' of heavy chain. I have seen people slip with 300 if they are anchored in heavy current and trying to get the hook to hold in the sand. The rule of thumb for rope length is 7 to 10 times the depth. Even fishing around trojan for fall chinook in 45-50', 300 was sometimes not enough.
Again, never try to pull your anchor by tying on to the stern. A boat went down in the Willamette last weekend trying to free up a stuck anchor by tying on to the stern.
Keep a knife handy for emergencies. An anchor and some rope are no reason to go for a swim.