Living in the UK we are certainly subject to the vagaries of changeable weather. When planning a swim, always check the forecast and be prepared for it to be wrong!
Because the sea covers a far larger area than any inland water, the effects of weather are correspondingly more extreme. Waves are formed by wind and can take days to cross an ocean, combining with waves from other directions as they travel.
The size of waves is controlled by wind speed, how long the wind blows for, and the area over which the wind acts. A big storm in the atlantic will lead to big waves hitting the west coast of the UK. Because so many factors impact on them over distance and time, waves are never entirely predictable.
Tides can also be affected by weather; a big storm will push water along with it in a storm surge, and combined with a spring high tide can cause catastrophic damage.
Areas affected by big tides are also subject to currents that vary in strength and direction. In some areas, the tidal stream can travel at speeds far faster than a swimmer can travel. Exposed headlands thus tend to be dangerous places to swim.
Local Wind will affect waves and make them more unpredictable. Messy surf tends to be blown out by an offshore wind (one blowing out to sea). It can also lead to wind chop or a roughing up of the surface, and often combines with swell and waves to cause difficult conditions for swimmers.