Most pool swimmers underestimate sea swimming because the variable they're used to — water temperature — is only one of four. The others are tide, current, and wind, and on a bad day any of them can turn a planned 1-km swim into a serious problem. The good news: each is predictable hours ahead with free tools.
Tides are the rise and fall of sea level driven by the moon. Every coastal swim spot has a tide chart (search "tide times [spot name]" — most local lifeguard or surfing sites publish them). The numbers you care about are *high water* and *low water* times, and the difference between them (the *range*). For open water swimming, slack water — roughly the hour around high or low tide — is golden. The water isn't moving, so you're not fighting horizontal current.
The hours *between* tides are when sea water moves fastest, sometimes at walking pace. A "weak" 2-knot tidal current is roughly 3.7 km/h — faster than you swim. If you fight one, you lose. Plan your swim to be in the water at slack, or to swim *with* the tide rather than against it.
Currents are more local. The two that matter for swimmers are rip currents (narrow flows of water heading offshore, usually through a gap in a sandbar) and longshore drift (water moving parallel to the beach). Rips are dangerous because they're strong and seem to "pull you out" — but the escape is simple: swim *parallel* to the shore until you're out of the rip's narrow channel, *then* swim in. Don't fight it directly.
You can spot rips by looking for darker, calmer-looking water flanked by breaking waves — the rip itself is deeper so it doesn't break. If in doubt at a new spot, ask a lifeguard or a local surfer.
Wind is the wildcard. Onshore wind (blowing from sea to land) makes the water choppy and uncomfortable but tends to push you back if you struggle. Offshore wind (land to sea) is the dangerous one — the water near shore looks deceptively flat, but the wind is pushing you out and the waves further out can be huge. New swimmers should never be in the water with an offshore wind above ~15 knots, no matter how calm it looks from the beach.
The decision rule: check tide times (plan around slack), check the wind direction and speed (onshore ≤ 20 kts is usually OK; offshore > 10 kts, reconsider), and look at the water for 10 minutes before getting in. Most "I had no idea" sea-swimming incidents could be avoided by a tide app and ten minutes of standing on the beach.