Beach conditions differ enormously between coastlines, and advice generalised across them is frequently wrong for two out of three.
The differences are driven by water temperature, wave energy, tidal range and seasonal hazards, and knowing which applies where changes what to expect.
Water temperature
The most immediately noticeable difference.
Gulf waters are warm through much of the year, warm enough for extended swimming across a long season.
Atlantic waters vary enormously by latitude, with the warm current influencing the southeastern coast and considerably colder conditions further north.
Pacific waters along much of the western coast are cold year-round, colder than the latitude suggests, because a cold current runs southward along the coast and because upwelling brings deep water to the surface.
Which is why the Pacific coast has spectacular beaches that are not swimming beaches in the way visitors expect, and why wetsuits are normal there and unusual elsewhere.
Wave energy
The second major difference and the one affecting safety most.
The Pacific receives long-period swell generated across an enormous fetch, producing powerful waves and strong currents.
The Atlantic receives significant swell, particularly during storm seasons.
The Gulf is comparatively enclosed, producing generally smaller waves outside storm conditions, which makes it the gentlest of the three for swimming.
Higher energy means stronger rip currents, and rip currents are the leading cause of surf rescues on any coast.
The seasonal hazards
Which differ enough to determine when to visit.
Tropical storm season affects the Gulf and Atlantic coasts, running through the late summer and autumn, with implications for travel planning and insurance.
Certain algal blooms affect Gulf coasts periodically, producing respiratory irritation onshore as well as water quality issues, and their occurrence varies year to year.
Jellyfish and other marine organisms have seasonal patterns that differ by coast and are locally known.
And rip current risk peaks under particular wind and swell conditions rather than following a season, which is why daily local forecasts matter more than general timing.
Tidal range
A difference that affects what the beach physically is.
Tidal range varies from modest in parts of the Gulf to substantial on some Atlantic and Pacific stretches.
Large range means the beach changes dramatically through the day, with implications for walking, for rock pools and for getting cut off around headlands.
Small range means the beach is broadly the same at any hour, which is simpler and less interesting.
The beach material
Which follows from the local geology and affects comfort directly.
Fine white sand in parts of the Gulf, produced from quartz eroded over enormous distances.
Varied Atlantic beaches ranging from fine sand to coarse and shingle depending on the local rock.
And Pacific beaches frequently coarser and darker, with more rocky shoreline and fewer long sandy stretches in many areas.
What this means for planning
The practical conclusions.
For warm-water swimming with gentle conditions, the Gulf in the shoulder months either side of summer.
For a mix, the southern Atlantic coast, with attention to storm season.
For landscape, walking and dramatic coastline rather than swimming, the Pacific, with the expectation that the water is cold and powerful.
And in every case, checking the local surf and rip current forecast rather than relying on the general impression, since conditions on a given day vary far more than the coastline's general character.
The safety point that applies everywhere
Swimming where lifeguards are present is the single strongest predictor of safety, and the difference in incident rates between guarded and unguarded beaches is substantial.
Which matters most on the higher-energy coasts and applies on all of them, and it is worth choosing a beach on that basis rather than on appearance.
Reading a rip current
The single most useful skill for any of these coasts.
Look for a channel of darker, calmer-looking water cutting through the breaking waves, frequently with foam or debris moving seaward.
The gap in the surf line is the danger rather than the safer route, which is the opposite of what it appears.
Standing and watching for several minutes before entering makes the pattern visible, and it is worth doing every time rather than on unfamiliar beaches only.
Local guidance on what to do if caught should be followed over any general advice.
Sun exposure by coast
A difference worth noting.
Ultraviolet intensity increases toward the equator and with altitude, and reflection from sand and water increases exposure substantially anywhere.
Cooler coasts produce more burns rather than fewer, because the sensation of heat is absent while the exposure continues, and wind removes the discomfort that would otherwise prompt shade.
Which is why the Pacific coast, with cold water and frequent breeze, produces burns in people who would have sought shade on a hot Gulf beach at the same exposure.
The seasons are offset
A planning point that follows from the temperature differences.
Peak swimming conditions arrive at different times on each coast, and the best month for one is frequently not the best for another.
The Gulf is usable across a long season and is unpleasantly hot and humid at the height of summer.
Northern Atlantic beaches have a short window concentrated in late summer, when the water has finally warmed.
And Pacific conditions are driven more by fog and swell than by calendar temperature, with late summer and early autumn generally clearest.