Note: I am not a lawyer.

You’re legally required to help, not risk your own life while doing so. If somebody is drowning you need to call the emergency services and if there is a rescue buoy close by you can throw that as well and try to pull them to shore that way.

I am not a lawyer, but I think in the majority of cases you should be covered by calling the emergency services.

As a side note: unless you’re a lifeguard or have had proper training, jumping in to save somebody drowning is dangerous. A person drowning will try to hold on to anything to stay afloat and will likely try to push you under trying to stay above water.