I have a Chrysler 318 engine in my boat with about 600 hours on it. It suddenly started burning oil in one bank. I'm thinking a bad valve guide or bad piston oil ring. What tests can I do to determine the problem?

Need to do a compression test. Both wet and dry will determine which is the culprit.
First do a dry test. Then when you find the bad cylinder(S) squirt a few drops of oil into the cylinder and repeat the test. If compression goes back up, then it is rings. If it stays low then it is valves.

Thanks for your answer, but I don't have a compression issue, I have an oil issue.

Thanks for your answer, but I don't have a compression issue, I have an oil issue.

Oil rings and valve guides don't usually "suddenly start burning oil." They wear gradually and it's usually the same or near the same on both banks. How do you know it's on one bank? From the plugs?

The only thing I can think of offhand that would suddenly provide more oil to a cylinder is the valve stem seal. If it cracks, oil can get sucked down the guide. But even then, it usually wouldn't happen to an entire bank at one. Time.

I second 450donn's advice... first do a compression check. At least rule that out. If you find compression lower on one bank, then you'll have a better idea.

Wear on one bank means a cooling problem.

Thanks for your answer, but I don't have a compression issue, I have an oil issue.

You have an oil issue in the cylinders? Right?
So that means that you have a compression issue. If you have a failed oil ring then only way of determining that is like I said . Either that or tear the engine down and do a complete inspection.

You have an oil issue in the cylinders? Right??
So that means that you have a compression issue. If you have a failed oil ring then only way of determining that is like i said . Either that or tear the engine down and do a complete inspection.

A cylinder leak test may be a better option before a total disassembly.
By properly setting the engine and introducing compressed air into the cylinders, one by one, you can determine where the loss is going:
Air escaping from the exhaust pipe would indicate a leaking exhaust valve.
Air escaping through the intake system would indicate a leaking intake valve.
From the oil dipstick tube would be an indication that the compression and/or oil rings are leaking.
Air escaping or bubbles in the radiator would obviously be from a failed head gasket or faulty cylinder head or block.

But, before I did any of that, I would thoroughly inspect the PCV system and make certain that the oil level was at the proper level.