No, I haven't any experience trying to build a pond in a wetland, but I do have some experience with wetlands. If you want to seriously pursue this you should seek some opinion from those who have everyday dealings with the subject...not to diminish the thoughts and experiences of the comments above for its a complicated subject. Not that these agencies have any regulatory authority, but you might seek thoughts from your local soil and water conservation district, local NRCS district conservationist, and state agency responsible for environmental quality - just for educational purposes. And, whatever you may discover, they are only the opinions of knowledgeable people outside the final determining authority.
The Army Corp may have regulatory authority. It just depends where this purposed piece of land is located in relation to potentially navigable waters, tributaries to those waters and the marshes inseparable from them. If the location is in a regulated area you still may be able to accomplish your objective, depending on what it is. Clearing land and building a dam? Probably not. Cutting trees and leaving stumps, maybe. Digging some dirt might depend on several factors. As I understand it, before Army Corp can issue a determination you must apply for a permit and present a proposal. Therein lies the rub. There are independent, for-hire wetland specialists who can help with that process. Is it worth it? I dunno. Unfortunately, or vice-versa depending on your view, the law does not differential between the big projects and the small. However, I've worked with some Corp people who have provided enough leniency to allow projects that would not get approved if proposed on a large scale.
That's my free advice and what it's worth given the price.
The issue of farming a defined wetland is governed by a whole different set of regulations - and it is possible to do it if one wants to forfeit any benefits paid by USDA in support of farm policy.