docs: add water quality integration guide - no API key required

- Document EU Bathing Water Quality API (free, no key needed)
- Add EPA Ireland and Marine Institute alternatives
- Provide implementation examples
- Clarify that water quality data is completely free and public

server/EXTERNAL_API_IMPLEMENTATION.md

@@ -250,10 +250,15 @@ redis-cli ping  # Should return PONG
 ```
 
 ### To Implement EPA Water Quality
-1. Research EPA Ireland API or data access methods
-2. May require web scraping or manual data entry
-3. Alternative: Use EU Bathing Water Quality database
-4. Update `getWaterQualityData()` function
+1. **Recommended:** Use EU Bathing Water Quality API (free, no key required)
+   - Endpoint: `https://www.eea.europa.eu/api/EMIS/get_bathing_water_data?country=IE`
+   - No authentication needed
+   - Covers all Irish designated bathing waters
+2. Alternative: EPA Ireland GeoPortal (manual download or web scraping)
+3. Alternative: Marine Institute Ireland data portal
+4. Update `getWaterQualityData()` function in `externalDataService.ts`
+
+**Note:** No API key required for any water quality data source
 
 ### To Implement Beach Quality
 1. Create static database of Irish beaches