Oarweed (Laminaria digitata) probably gets its common name from its shape but given it can form large masses of entangled weed it may have got its name from oarsmen having problems rowing through it with it catching their oars in the water! My reference material is not clear on the subject.
Oarweed certainly resembles a large paddle. It has a long, broad 'blade' up to 4 feet long and a shorter 'handle' about a foot long. The blades are large leathery structures that collapse under their own weight at low tide in to a slimy, slippery blanket on rocks making them perilous to walk across. They form large colonies and provide shelter for many other underwater creatures and plants.
Common around rocky coasts and not uncommon in Dorset.Laminaria digitata: oarweed