Luggage and bags: Must have a good water resistant backpack or a backpack/drybag.
See-through plastic drybags make it easier to get at your stuff.
A mesh dive bag is handy to carry your wet stuff or snorkel gear.
A fanny pack or small day pack for hiking trips.
Clothing/Shoes/Weather Gear: Kayak shoes or dive booties are best for kayaking. Sport sandals are OK, but your feet get sandy & gritty.
A good pair of hiking shoes or high-top sneakers are best for hiking in the desert. There's lots of prickly stuff on the ground, so you don't want your ankles exposed.
Bring a wide-brimmed hat.
I used a long sleeve light-colored biking shirt for paddling, which worked great. It was light enough so I didn't get too hot and it was enough to block the sun from my arms.
Some bike gloves or kayak gloves will help prevent blisters on your hands.
Even when it's hot during the day, you'll need a sweatshirt and a wind-breaker at night.
Toiletries and Medical Supplies: Tweezers & nail clippers! Make sure you put them in your checked baggage because you cannot carry them on the plane.
Ear-drops if you're snorkeling
Immodium - I didn't need it but some people did. You don't want those kinds of problems when you're in a kayak. Better safe than sorry.
Tums or Rolaids or something for heartburn. It's Mexico after all and dehydration makes it worse. I forgot it and regretted it.
Sunblock of SPF 25 or higher. 'Sport' formulas are better in the water.
Some sort of lip balm with sunblock.
Biodegradable soap that lathers in salt water. You can find some at http://www.campmore.com
Photo Equipment: A small drybag or drybox is handy to stow cameras or other gear you want to take with you in the kayak but can't get wet.
Bring Q-tips or lens tissue to clean off your lenses. It gets pretty dusty.
A disposable underwater camera for snorkeling with sea lions. Sadly, I forgot to bring one :>(
Camping/Beach/Outdoor Gear: One of those small wrap-around-your-neck airline pillows works well for camping when you need to pack light.
A water bottle is a must.
Miscellaneous: Do not forget ear pads if your hotel is on the Malecon (most of hotels are there); it is the busiest avenue and, especially on weekends, the traffic is extremely noisy. But don’t ask for a room without the view of Malecon for this reason because it’s breathtaking, especially on sunsets.