The whole reason people come to Port Aransas is the water. It is where the island earns its living and how most visitors decide whether a trip was worth it — was the Gulf clean, was it warm enough to swim, was the beach clear of seaweed and free of anything that would sting your eyes or your lungs? Those are simple questions with surprisingly good public answers, because the same water that draws the crowds is monitored week in and week out by state and federal agencies. This report gathers that monitoring into one place, for one island, and refreshes it as new readings are published.

We report three things, and only what the data actually supports. First, how clean the water is: the Texas Beach Watch program samples Port Aransas and Mustang Island beaches for Enterococcus bacteria — the standard indicator the EPA uses to flag water that could make swimmers sick — and we summarize those results as a plain safe-sample rate against the federal threshold. Second, how warm the Gulf runs through the year, using a multi-year temperature record from the NOAA tide station at the mouth of the ship channel. Third, when the two seasonal nuisances every islander knows — red tide and sargassum seaweed — typically show up, drawn from the agencies that track them.

A note on method, because it matters here more than anywhere: there is no secret sauce and no black-box "water quality score." The safe-sample rate is just the share of real samples that met a published EPA standard, and we show you the sample counts behind it. Water temperature is an average of real hourly readings. Where a month has never been sampled, we leave a gap rather than draw a zero. And the red tide and sargassum windows below are described as typical seasonal patterns attributed to their sources — never as a forecast for any particular week or year, and never with a made-up number attached.

The headline numbers

Safe-sample rate

92.5%

of samples met the EPA standard · 2021–2025

Samples analyzed

3,044

Beach Watch Enterococcus tests · 228 over threshold

Warmest month

87°F

Average Gulf temperature in Aug · 60°F in Feb

How clean is the water, month by month

Every bar below is the share of Beach Watch samples taken at Port Aransas and Mustang Island beaches that came in at or below the EPA's single-sample beach action value of 104 colony-forming units of Enterococcus per 100 mL. A sample at or under that line meets the federal recreational-water standard; a sample above it is what triggers a swimming advisory. This is the entire calculation — a pass rate against a published threshold, with the sample counts shown in the table underneath. There is no weighting and no composite index.

Month	Safe-sample rate	Geometric mean	Samples
Jan	96.6%	9 cfu/100 mL	206
Feb	100%	5.3 cfu/100 mL	152
Mar	96%	9 cfu/100 mL	353
Apr	90.5%	15.3 cfu/100 mL	200
May	84.6%	18.9 cfu/100 mL	384
Jun	84%	23.4 cfu/100 mL	376
Jul	91.4%	20.3 cfu/100 mL	349
Aug	93%	12.5 cfu/100 mL	257
Sep	92.7%	13 cfu/100 mL	205
Oct	99.1%	8.3 cfu/100 mL	232
Nov	100%	6.8 cfu/100 mL	160
Dec	97.6%	6.1 cfu/100 mL	170

Geometric mean is the standard summary statistic for bacteria counts; it is shown here alongside the pass rate for transparency and is not part of the safe-sample calculation. Across all sampled months the overall geometric mean is 12.4 cfu/100 mL.

When the Gulf is warm enough to swim

This is the seasonal temperature curve of the Gulf at Port Aransas, built from real hourly readings at the NOAA tide station by the ship channel over 2021–2025. The line is the monthly average; the shaded band shows the typical range readings fall within (the 10th to 90th percentile), so you can see not just the average but how much a given month swings. It is a climatology — a picture of a normal year — not a prediction for any specific date.

The two seasonal nuisances, and when they show up

Two natural phenomena shape a Port Aransas beach day more than anything the water-quality lab measures: red tide, the algae bloom that can irritate eyes and airways and litter the sand with dead fish, and sargassum, the brown Gulf seaweed that piles up along the wrack line. Neither is a pollution problem and neither happens on a schedule you can bank on. What the tracking agencies can tell you is the typical season — the months in which each is most likely. The strips below mark those typical windows. They are seasonal norms, not a forecast, and carry no invented magnitude.

Red tide (Karenia brevis)

Typical peak season: August through November

Typical seasonal pattern, not a forecast. Source: Texas Parks & Wildlife Department — Red Tide

Sargassum (Gulf seaweed)

Typical peak season: April through July

Typical seasonal pattern, not a forecast. Source: USF Optical Oceanography Lab (Sargassum Watch) & UT Marine Science Institute

For a plain-language rundown of what to do when the seaweed rolls in, see our Port Aransas sargassum & seaweed guide.

Sources & methodology

This report is assembled entirely from public monitoring data and refreshed on a recurring basis. Water-quality figures come from the EPA Water Quality Portal, the authoritative national aggregator that carries the Texas Beach Watch Enterococcus results; we keep only the Port Aransas and Mustang Island beach stations, aggregate by calendar month across several years, and report the share of samples at or below the EPA single-sample action value of 104 cfu/100 mL, with sample counts shown. Water temperature is a climatology of real hourly readings from NOAA CO-OPS station 8775237 at Port Aransas, averaged by month with a 10th-to-90th-percentile band. Where a month has no samples or readings, we leave it blank rather than fill it with a zero. Red tide and sargassum are presented only as typical seasonal windows attributed to the agencies that track them — never as a forecast or an invented number.

NOAA CO-OPS — Port Aransas water temperature (station 8775237) — last pulled July 20, 2026
EPA Water Quality Portal — Texas Beach Watch Enterococcus samples (Port Aransas & Mustang Island) — last pulled July 20, 2026

Have a correction or a data source we should add? Get in touch. See more data-driven reporting on the Research hub.