The lake at Wakefield's Pugneys Country Park has been closed after high levels of bacteria were discovered in the water.
Wakefield Council said watersports were suspended and warned people that they must stay out of the lake.
The authority said high levels of bacteria were found during routine water testing.
The bacteria can cause serious illness.
A statement on the council's website states: "As a precautionary measure, watersports at Pugneys have been temporarily suspended and people must keep out of the water.
"Dog owners are also being asked to ensure their pets stay away from the lake.
"We will be working with partner agencies to identify the source of the bacteria and ensure action is taken as soon as possible.
"In the meantime, water testing will continue and we will keep people informed when the situation changes.
"We're sorry for any inconvenience."