From Discworld MUD Wiki
These shoes spread out into wide flat flippers. While they would be very awkward to walk around in on land, they might be useful in the water - or at least more fitting.
The pair of fishy flippers is about a foot long and about six inches wide. It is green and is made of rubber and looks like you could wear it.
The pair of fishy flippers is 1 foot long and 6 inches wide.
The pair of fishy flippers weighs 1 1/9 lb.
They are sold in the shop of the sunken ship below
While in water it should help move around.
The swimming bonus doesn't change when putting it while underwater though.
In a room with water:
wear flippers You feel appropriately dressed for moving around in the water. You wear a pair of fishy flippers. remove flippers Upon removing the flippers you feel less fishy. You remove the pair of fishy flippers.
wear flippers You feel rather odd wearing flippers while not moving around in water. You wear a pair of fishy flippers. remove flippers Upon removing the flippers you feel a certain relief. You remove the pair of fishy flippers.