Super Swim is a glitch that allows Link to walk through walls. Most main categories ban it, however many unrestricted categories (like All Instruments or 100% No Restrictions) use it extensively.
A Super Swim is essentially a super jump done while swimming.
If done correctly, Link will make a hopping noise and swim away from the wall - however no animation will occur. Link will now be able to walk onto any tile you choose.
Note that transitioning downwards will cause the game to crash.