Do I have to peel rhubarb first?

Usually no. Just wash and chop. If the outside looks very tough or stringy, you can peel a strip or two, but it is not common.

Why is my rhubarb crisp watery?

Most of the time it needs more bake time. You want real bubbling. Also measure the cornstarch and let it cool a bit before serving so it thickens.

Yes, but keep in mind rhubarb is naturally very tart. I would not go below 1/3 cup sugar unless you are adding strawberries or another sweet fruit.

You can, but old fashioned oats give you the best texture. Quick oats make the topping finer and a little less crunchy.

An 8 by 8 inch dish is perfect for a standard batch. If you use a larger pan, the crisp will be thinner and may bake faster.

Scale

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Heat oven to 375 F.
Lightly butter your baking dish.
Mix the rhubarb filling in a bowl and pour it into the dish, spreading it out evenly.
Mix the topping in a separate bowl until crumbly.
Sprinkle topping evenly over the rhubarb. Do not press it down.
Bake for 35 to 45 minutes, until the top is deep golden and the filling is bubbling around the edges.
Cool for 15 to 20 minutes before serving to let the filling thicken.

Serve warm with vanilla ice cream or whipped cream for a delightful treat.