| Magic Island lagoon | Hawaii | Coastal Waters | 26.9°C(80.4°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Mahaiula Beach | Hawaii | Coastal Waters | 27.1°C(80.8°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Maidstone Park Beach | New York | James Bay | 20.4°C(68.7°F) | July, August, September, October | Live SST → |
| Main Beach | California | Gulf of California | 20.8°C(69.4°F) | July, August, September, October | Live SST → |
| Mākaha Beach | Hawaii | Coastal Waters | 27°C(80.6°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Makalawena Beach | Hawaii | Coastal Waters | 27.1°C(80.8°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Makamah Beach | New York | James Bay | 21.5°C(70.7°F) | June, July, August, September | Live SST → |
| Makapuu Beach Park | Hawaii | Coastal Waters | 26.8°C(80.2°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Makolea Beach | Hawaii | Coastal Waters | 27.1°C(80.8°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Mākua Beach | Hawaii | Coastal Waters | 26.8°C(80.2°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Malibu (Zuma Beach) | California | Pacific Ocean | 19.8°C(67.6°F) | Jul, Aug, Sep | Live SST → |
| Malibu Beach | California | Gulf of California | 20.2°C(68.4°F) | July, August, September, October | Live SST → |
| Maluaka Beach | Hawaii | Coastal Waters | 26.8°C(80.2°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Mamanasco Beach | Connecticut | James Bay | 22.3°C(72.1°F) | July, August, September | Live SST → |
| Manatee County Beach | Florida | Gulf of Mexico | 29.6°C(85.3°F) | May, June, July, August, September, October | Live SST → |
| Mandalay Beach | California | Gulf of California | 19.4°C(66.9°F) | July, August, September, October | Live SST → |
| Mandalay State Beach | California | Gulf of California | 19.2°C(66.6°F) | July, August, September, October | Live SST → |
| Manhattan Beach | California | Gulf of California | 14.9°C(58.8°F) | July, August, September, October | Live SST → |
| Manhattan Beach | New York | James Bay | 21.8°C(71.2°F) | June, July, August, September | Live SST → |
| Mansion Beach | Rhode Island | James Bay | 19.9°C(67.8°F) | July, August, September, October | Live SST → |
| Maple Street Beach | Illinois | James Bay | 20.1°C(68.2°F) | July, August, September, October | Live SST → |
| Marconi Beach | Massachusetts | James Bay | 18.3°C(64.9°F) | June, July, August, September | Live SST → |
| Margaret Burroughs Beach | Illinois | James Bay | 20.2°C(68.4°F) | July, August, September, October | Live SST → |
| Marine Street Beach | California | Gulf of California | 21.2°C(70.2°F) | July, August, September, October | Live SST → |
| Markey Beach | United States | James Bay | 14.2°C(57.6°F) | July, August, September, October | Live SST → |
| Marlboro Beach | Maine | James Bay | 12.8°C(55°F) | July, August, September, October | Live SST → |
| Marshall's Beach | California | Coastal Waters | 14.5°C(58.1°F) | July, August, September, October | Live SST → |
| Marshalls Beach | Virginia | Coastal Waters | 24.9°C(76.8°F) | July, August, September | Live SST → |
| Martins Beach | California | Gulf of California | 14.9°C(58.8°F) | July, August, September, October | Live SST → |
| Marvin Beach | Connecticut | James Bay | 22.3°C(72.1°F) | July, August, September | Live SST → |
| Mashes Sands Beach | Florida | Gulf of Mexico | 29°C(84.2°F) | May, June, July, August, September, October | Live SST → |
| Matagorda Beach | Texas | Gulf of Mexico | 29.4°C(84.9°F) | May, June, July, August, September, October | Live SST → |
| Matthew J. Buono Beach | New York | James Bay | 21.8°C(71.2°F) | June, July, August, September | Live SST → |
| Matunuck Beach | Rhode Island | James Bay | 19.7°C(67.5°F) | July, August, September, October | Live SST → |
| Mauumae Beach | Hawaii | Coastal Waters | 27°C(80.6°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Maxwell Place Beach | New Jersey | James Bay | 21.8°C(71.2°F) | June, July, August, September | Live SST → |
| McAbee Beach | California | Gulf of California | 14.9°C(58.8°F) | August, September, October, November | Live SST → |
| McCook Park Beach | Connecticut | James Bay | 20.4°C(68.7°F) | July, August, September, October | Live SST → |
| McKinley Beach | Wisconsin | James Bay | 18.9°C(66°F) | July, August, September, October | Live SST → |
| Medeiros Beach | Massachusetts | James Bay | 20.2°C(68.4°F) | June, July, August, September | Live SST → |
| Medrick Beach | United States | James Bay | 14.2°C(57.6°F) | July, August, September, October | Live SST → |
| Menahaunt Beach | Massachusetts | James Bay | 20.2°C(68.4°F) | June, July, August, September | Live SST → |
| Menemsha Beach | Massachusetts | James Bay | 19.9°C(67.8°F) | July, August, September, October | Live SST → |
| Meudon Beach | New York | James Bay | 21.7°C(71.1°F) | June, July, August, September | Live SST → |
| Miami Beach | Florida | Atlantic Ocean | 29.8°C(85.6°F) | May, Jun, Jul, Aug, Sep, Oct | Live SST → |
| Miami Beach | United States | Gulf of Mexico | 29.5°C(85.1°F) | January, February, March, April, May, June, July, August, September, October, November, December | Live SST → |
| Miami Beach | Maryland | Coastal Waters | 24.1°C(75.4°F) | July, August, September | Live SST → |
| Middle Beach | Connecticut | James Bay | 20.9°C(69.6°F) | July, August, September, October | Live SST → |
| Middle Beach | United States | James Bay | 13.5°C(56.3°F) | July, August, September, October | Live SST → |
| Midland Beach | New York | James Bay | 21.8°C(71.2°F) | June, July, August, September | Live SST → |