The latest Coronavirus (COVID-19): guidance and support includes the following paragraph

All forms of water sports practiced on open waterways, including sailing, windsurfing, canoeing, rowing, kayaking, surfing, paddle-boarding and the use of privately-owned motorised craft (in line with the guidance issued by the relevant navigation authority) are allowed. You can continue to use towpaths for walking, running and cycling, being mindful of other users and people living in boats along the water.

Read the full guidance here.