Marriott's is not a very difficult rapid, however you have to choose your line and paddle convincingly to avoid some of the messy stuff in the centre. The rapid has two parts to it, namely a drop on the bend as the river turns right and then a rocky, fairly flat section with large boulders strewn about it. The second part has a gradual gradient, so you can read the lines well. The easiest option is to hug the inside of the first bend (keep right) and then move back into the centre as the river opens up again.
In low conditions stick to the main current and follow the flow. Be prepared for some tight turning.
In very high conditions it gets easier and the lines open up, but best to avoid the centre and left of the first bend where a massive pour over develops. There is a safe and easier line if you keep extreme right.