Recommendation from the B4D Forum:

For those of you interested in stand up paddle and/or kayaking, please contact Dirceu. He speaks English and is a very friendly guy.

Dirceu is located in SHTN Asa Norte but for a small surcharge he can deliver the boards to any place around the lake. He charges R$30 an hour per board and offers free instructions for beginners.

Unfortunately they only operate on weekends and public holidays.

Update: They now have an Instagram profile.