Mexico Beaches Best

The best months to go to Mexico beaches are November - May. 
July-August, Christmas, and Easter attract huge numbers of holidaymakers from both inside and outside the country, especially at beach resorts. The worst are the hottest and most humid months, June - September, with an insect problem too.
Aug-Nov has the possibility of rough seas on the Pacific coast due to storms.
For diving off Baja California August - November is best, with warm, clear water.
Plus: great varied food and drink are generally available, lively evenings with music and dance are common, coral is superb in some places, some nearby ancient sites are AMAZING, such as Chichen Itza and beach-side Tulum, while Spanish is an easy language to pick up a few words or more.
Minus: it's very package touristy in many places, souvenirs are poor, petty theft happens and local service people can be sour and unhelpful if things go wrong or tips are not evident. 

Mexico's beaches can be divided into three areas, Gulf Coast, the Maya Riviera [Caribbean Coast] and the Pacific coast.

Gulf of Mexico coast:
Not a lot of great beaches on this curving, tropical shore, though plenty of oil wells.
The state of Veracruz occupies a large part of the coast and is scattered with still unspoilt towns and archeological sites as this is a less-travelled part of the country.
The state's best beaches run along the Costa Esmeralda, a 31 mile [50km] strip north of the Veracruz port [5 hours drive from Mexico City]. The sand tends to be grey-brown, waters warm and large stretches are devoid of crowds so this is one of the better places in the country to find isolation. Nearby is the UNESCO World Heritage site of El Tajin, a ceremonial center of the Totonacs.
Mayan Riviera [the Caribbean coast]:
Puerto Morelos is a fishing village and small beach resort between Cancun and Playa del Carmen with fine sand, warm water and for a great bonus, the Mesoamerican Barrier Reef just 500 meters offshore for snorkelling or diving...more
Playa del Carmen
A cool, casual beach scene with plenty of accommodation [though there's an increasing imbalance towards huge, all-inclusive hotels these days], excellent bars, restaurants and shops. Although it is very popular Playa del Carmen is reasonably low-key and relaxing.
This is one of the better places in Mexico to buy high quality crafts, especially jewellery.
Playa also has boats to Cozumel island - for world class diving, pricey shopping [catering to cruise ships], a couple of fine beaches and a marine park for swimming with dolphins - at considerable expense. See Playa del Carmen Video.

Tulum beach and ruins, Maya Riviera, Mexico

Tulum
Tulum is one of the prettiest beaches in the Americas, with picturesque Mayan ruins overlooking the sand.
130 km south of the massive and style-free resort of Cancún, it is a tiny but perfectly-formed white sandy cove, with a few palm shade and the azure Caribbean sea to swim in.
The entrance to Tulum is totally touristy, with a huge car park, souvenir shops, café and restaurants. There are a few hotels and restaurants in Tulum village and also [far better option] plenty of resort type places, cabañas [beach huts] and campsites along the coast road south to Punta Allen.
Mexico's Pacific shore offers the greatest variety of beaches and can be divided into two parts - Baja California, a long peninsula bordering the USA's California [half washed by the North Pacific Ocean, the other half by the Sea of Cortez] and the more southerly mainland Pacific Coast.
Baja California [peninsula]:
Los Cabos, Mexico- [Cabo San Lucas {photo at top} and San Jose del Cabo]
Offering miles of marvelous beaches at the tip of the baja peninsula this is a major American tourist destination for both fishing and golfing older folk or tanning and tequila young party animals.
The best Cabos beaches are:
Playa Cemeterio 4 km east, Playa Santa Maria 12 km from Cabo San Lucas for swimming, with easy access.
Playa Costa Azul, Playa Canta Mar are ideal for surfing.
Playa Solmar, 1 km west of Cabo San Lucas, is for whale- watching January - April.
Scuba diving and snorkelling are OK, but the the best sites are offshore, reachable only by boat.
Infamous time-share salesmen can be annoying, don't believe a word they say! And beware strong rip tides at some of the beaches..Photos | Cabo San Lucas Map
Los Cabos Weather:
Cabo San Lucas weather doesn't vary a lot with generally little rain and moderate heat apart from late summer tropical storms that can dampen the atmosphere, cause cloud, seaweed-strewn beaches and poor water visibility for snorkelers and divers. However, often the storms occur out to sea or in the mountains in the Los Cabos region and will have little effect on beach resorts.
Temperatures range from January lows-highs of 58F-77F [14C-25C] to August lows-highs of 81F-95F [27C-35C].
Note that Los Cabos are a popular 'Spring Break' location so muchos locos descend upon the area around March every year and tranquility may be hard to find. Christmas, New Year and Easter school holidays are also busy but not so crazy.
Bahía Concepción, near La Paz
The shore of Bahia Concepcion [Conception Bay], just south of Mulegé [near La Paz], has several beaches that many consider are the best in the Baja, including Playa Requesón, Playa Buenaventura, and Playa Coyote. The bay is on the coast of Sea of Cortez, which is a protected marine sanctuary and offers quieter, warmer waters than the Pacific side.
The facilities are fairly basic [some places have no fresh water], but there is wildness, natural beauty and tranquility. Excellent for watersports and great for seafood, especially clams. Playa Santispac has the best campsite, with a fine restaurant. If you are up to kayaking, try Playa Buenaventura.
The Pacific mainland coast, from north to south:
Mazatlan is a vibrant port town, both a transport hub with ferries to Baja California, and a beach destination with huge stretches of sand and warm waters. Mazatlan is a popular Spring Break destination due to its variety of low-cost accommodation. Photo.
Bahia de Matanchen, San Blas [ near-ish to Tepic]
This small and very Mexican fishing village of San Blas has an excellent town beach, but the best in the area is just 4 km away - Bahia de Matanchen, a broad crescent bay with splendid beaches of prime soft sand and a relaxing atmosphere.
The only drawback is summertime mosquito and sand fly problems.

Puerto Vallarta, Pacific Coast, Mexico

Between Puerto Vallarta and Acapulco
The central Pacific Coast has several hundred miles of coastline with fine sandy beaches and excellent facilities. The choices are enormous from international resorts like Puerto Vallarta and Acapulco for monster all-inclusive resorts packed densely, to smaller resorts with local colour such as: Chamela [the prettiest, 165 km south of Puerto Vallarta], Barra de Navidad and Zihuatanejo [picture below]. Great activities, fantastic seafood, and lively night life.

Puerto Escondido [San Pedro Mixtepec, Oaxaca state, SE of Acapulco]
Puerto Escondido has several stretches of magnificent sandy beach, with a laid-back ambience and plenty of inexpensive accommodation. Beaches range from 2 km long Zicatela, the surfing mecca [nicknamed the Mexican Pipeline] east of the town, to the calmer three coves including Playa Puerto Angelito on the west. August - November surf reaches 7-8 metre due to south Pacific storms.
Puerto Angel [San Pedro Pochutla, Oaxaca state, 50 miles east of Puerto Escondido]
For even more tranquility go 50 miles east to the Puerto Angel area, including the backpacker's favourite, Zipolite Beach, 4 kms from Puerto Angel [take a taxi] and about 60 km east of Puerto Escondido. Zipolite offers simplicity and relaxation [except for the overdeveloped west end], with a good selection of low-cost accommodation and eateries beside the sand, wifi too! Recently there has been an increasing security problem. Beware of high surf [particularly August -November] and strong currents in deep water. n.b. Surf board rentals are not easy to locate
Mazunte Beach, 10minutes further up the coast by taxi has a huge turtle reserve that is fascinating.