We went to Mexico a few years ago now. I like visiting and investigating places when I am away, as personally I don't see the point in travelling and not exploring some of the culture you may never see again.
One of the trips on offer from our hotel base was to Xel-Ha.
Xel-Ha is situated on the Riviera Maya which runs along the carribean coastline and is situated amoungst the Yucatan Peninsula.
To put this in perspective for travellers - Cancun is about 90 minutes away and Playa del Carmen 30 minutes.

SO WHAT IS XEL-HA - Xel-ha is a natural aquarium, granted built around, which is sympathetically done to enhance visitor experiance without disturbing to much of the ecological or biological area. The park has been developed amoungst the land where the river opens up and meets the ocean. Read on to hear more!.....

THE COST - as this will be ever changing please refer to the website www.xel-ha.com but at the time of writing it was $90 (£60) adults and $60 (£40) children. Now I know this may seem a little high, especially if you are a family, however, bear in mind this is mostly all inclusive and is an entire day visit. Open 365 days a year! It always opens at 0900 but closes at varying times over the year.
You can usually pick up a leaflet from your hotel or get one on the actual visit. This I feel is essential as it gives you a comprehensive guide of what is avaliable, where it is and a map of the grounds.

SO WHA CAN YOU DO? This section I will split up into the various things you can do, as their are so many!

SNORKELLING - When you first arrive at Xel-ha you immediatly notice the vast area of ocean and see people bobbing up and down in it. Now I have to say I love the water, but am a little nervous out in the open ocean, escpecially when there are unknow fish. Never one to shy from a challenge I saw a little child dipping in - so that was that off I went. When you get closer to the water edge you clearly see guys fitting people with life jackets which are inclusive. You also get the choice of snorkelling equipment which you pay a small depoist, returned when you take the items back (you get to keep the mouth piece). When fitted you can make your way off to a wooden pier with steps down into the water. This although covered with rubber type stuff on steps, can get slippy so beware. I was immediatley surprised at how many fish were right there! and big ones at that! Numerous varieties are around for you to see - parrot and angel fish, snappers and many other ones normally found in your tropical fish tank at home :) We even spotted a baracuda further out! It is fantastic to get that close to them and feel them bounce off your feet as you swim. It's essential to take an underwater camera if you can, even a novice like me got some fab pics.
All around the inlet water area are wooden beach loungers which are free so you can relax and watch everyone else, great for families.

SNUBA - We never tried this but it is apparently a cross between snorkelling and scuba diving. You don't take a oxygen tank, but are connected to a pipe which in turn runs to a boat above.

SEA TREK - Again this we didn't try unfortunatly. There is an extra charge and the queues we just far to long for us, so get there early. It entails where a helmet connected to a pipe line and you walk along in a guided tour a few feet underwater.

CENOTES - As you go further inland there are cenotes or sinkholes, these are open pools of water normally connected to caves over and under the water. These you cannot swim in but there are options to kayak over them, or simply walk around them and spot some great wildlife too.

FLOATING BRIDGE - Down where the people go sea trekking is the floaing bridge. This is a wide floating bridge across the ocean entrance to the other side of the inlet. Bearing in mind this floats it goes with the waves and can be very rocky! so beware if you have little children and keep a firm hand hold. It is fun though and the fish dash under and up to the sides in shoals.

THE WALKS - Although titled walks you can take a push bike and there are little shuttles to and from certain areas. We chose to walk. It is a long way so bear this in mind if you have children, of an older age or disabilities. Places are plentiful to stop and rest though throughout; varying from small steps to the water to seats and tables.
Fantastic scenery, wildlife, many birds, squirrels and loads and loads of iguanas which are hugh! We spotted seven in one particular place all sun bathing :)

INNER TUBES/SWING ROPE/WALKING ROPE AND CLIFF JUMP
Along the vast walks there are things to do. The inner tubs are large floating rubber rings which can be found at the start of the river area, which you can jump on and float back off down the river to the main inlet. Hugely relaxing after a long walk.
Also dotted about are the swing rope across the water, a walking rope which is a tight rope affair with another rope to hang onto, which goes over the water and the cliff jump. Relaxing isn't for everyone so why not try the jump off a large cliff top into the cool, clear water below.

LIGHTHOUSE/HAMMOCKS
For the more gentle of us there is a lighthouse on the rocks which is surrounded by beach and hammocks to lie in.

PLANTS/NURSERY
There are a lovely garden which is greenhouse type idea with an abundance of plants and the mayan bee area to visit.

DOLPHINS
There is an option to swim with dolphins at an extra cost over $100 each for hour. We didn't do this as we swam with them and other animals in another trip. You can walk around the dolphin area where they stay.
So thats the main attractions covered, here is some of the rest.....

PARROTS/IGUANAS
Guys come up to you every now and then offering to take your picture with parrots and iguanas which are at added cost.

LOCKER AND CHANGING areas are avaliable too, free towels, just watch out for the odd spider lurking!
I should point out here too that the park only allows you to wear bio friendly sun tan lotion, due the fish. This is pretty much expected everywhere in Mexico and the lotion is cheap and avaliable in most resorts.

FOOD/DRINK
The vast majority of food is included in the price and there are a range of food eating places, from buffets to set meals. There is also the option to buy fom smaller areas. All good food in my experiance and choices, I heard no one complaing.

STAFF
I had no issues with staff, they all seemed very helpful when asked things. Many spoke pretty good english and other languages. It is important to remember though that due to the vast area and small water areas, there are not always staff around in different parts; if you have children make sure you keep an eye on where they are wandering off too.

OVERALL
This is a fantastic place and I would go back in a hearbeat. You definatly need a whole day, so don't be tempted to join it up with the other visits, otherwise you would miss out on some things or rush to much and that would be a great shame.
Xel-ha is not only popular with tourists but also with locals, which I think says a lot if they are prepared to pay to visit it too.

Review maybe posted on dooyoo and ciao under the same user name.