In winter, whale-watching tours on the Pacific coast are popular excursions from Loreto-based resorts. Bahía Magdalena [1] is one of only three places along the Pacific where you can watch gray whales up close as they migrate into the shallow lagoon to care for their calves.
For diving and fishing, summer is the best time to go. But temperatures are more moderate in March–May and November–December. January and February are the coldest months, when the north winds blow. The region is quietest during the rainy season in September.
Links:
[1] http://www.moon.com/destinations/baja-cabo/loreto-and-vicinity/bahia-magdalena