Swimming and snorkeling in the ocean is not like swimming in a pool or lake. The ocean develops currents that can be very strong, actually irresistible, at times. Combine this with the shape of the shoreline and the presence of waves, and things like 'rip currents' can develop. These are currents that can carry a bather from the relative safety of the inshore waters out into deeper water, where possibly stronger currents may exist. So bathers sometimes walk into the surf unawares that a strong out-going current that can actually knock them off their feet and carry them out to sea, is right there.
Strong Currents develop when there is the presence of wind.
In Hawaii the Trade Winds are a nearly constant companion. The stronger the wind blows, the stronger a developing surface current might become. Generally these currents flow in the direction of the wind flow, but not always. The shape of the shoreline and the interaction of other deeper water currents can cause surprising changes in the overall current flow around the islands. When planning any activity, check to see if strong winds are forecast and what direction they might be in.