http://www.dnr.state.mn.us/shorelandmgmt/apg/regulations.html

When a permit is NOT needed
If you are a lakeshore-property owner who wants to create or maintain a swimming or boat-docking area, you may cut or pull submerged vegetation, such as Elodea, without a DNR permit under certain conditions:

• First, the area to be cleared must be no larger than 2,500 square feet.
• Second, the cleared area must not extend more than 50 feet along the shoreline or one-half the length of your shoreline, whichever is less.

A boat channel up to 15 feet wide, and as long as necessary to reach open water, may also be cleared. (The boat channel is in addition to the 2,500 square feet allowed). The cutting or pulling may be done by hand or with hand-operated or powered equipment that does not significantly alter the course, current, or cross-section of the lake bottom. Such control cannot be done with draglines, bulldozers, hydraulic jets, suction dredges, automated untended aquatic plant control devices, or other powered earth-moving equipment. After you have cut or pulled aquatic plants, you must dispose of them on land to prevent them from drifting onto your neighbor's property or washing back into the lake. In addition, a channel 15 feet wide through floating-leaf vegetation (other than yellow lotus, a protected wild flower) extending to open water may be maintained by mechanical means without a permit. Any other destruction of floating-leaf vegetation requires a permit. If you have any questions on control activities that do not require a permit, call your local DNR office.

A DNR permit is not needed to gather aquatic plants for personal use (except for wild rice and yellow lotus) or for constructing a shooting or observation blind.