The Great Barrier Reef is to Australia what rolling savannahs and game parks are to Africa, and is equally subject to the corniest of representations. "Another world" is the commonest cliché, which, while being completely true, doesn't begin to describe the feeling of donning mask and fins and coming face to face with extraordinary animals, shapes and colours. There's so little relationship to life above the surface that distinctions normally taken for granted – such as that between animal, plant and plain rock – seem blurred, while the respective roles of observer and observed are constantly challenged by shoals of curious fish following you about.

Beginning with Lady Elliot Island, out from Bundaberg, and extending 2300km north to New Guinea, the Barrier Reef follows the outer edge of Australia's continental plate, running closer to land as it moves north: while it's 300km to the main body from Gladstone, Cairns is barely 50km distant from the reef. Far from being a continuous, unified structure, the nature of the reef varies along its length: the majority is made up by an intricate maze of individual, disconnected patch reefs, which – especially in the southern sections – sometimes act as anchors for the formation of low sand islands known as cays; continental islands everywhere become ringed by fringing reefs; and northern sections form long ribbons. All of it, however, was built by one animal: the tiny coral polyp. Simple organisms, related to sea anemones, polyps grow together like building blocks to create modular colonies – corals – which form the framework of the reef's ecology by providing food, shelter and hunting grounds for larger, more mobile species. Around their walls and canyons flow a bewildering assortment of creatures: large rays and turtles "fly" effortlessly by, fish dodge between caves and coral branches, snails sift the sand for edibles, and brightly coloured nudibranchs dance above rocks.

The reef is administered by the Marine Parks Authority, which battles against – or at least attempts to gauge – the effects of overfishing, pollution, agricultural runoff, environmental fluctuations and tourism. All these things are beginning to have a serious effect on the Reef, with many formerly colourful coral gardens reduced to weed-strewn rubble. Don't let this put you off going – the Reef is still unquestionably worth seeing, and if the government realizes how much tourism will be lost if the Reef dies, they may get more involved in protecting it. In order to minimize damage, visitors should never stand on or hold onto reefs when snorkelling or diving; even if you don't break off branches, you'll certainly crush the delicate polyps.

Scuba diving is the best way to get to grips with the Reef, and dive courses are on offer right along the coast. Five days is the minimum needed to safely cover the course work – three days pool and theory, two days at sea – and secure you the all-important C-card. The quality of training and the price you pay vary; before signing up, ask others who have taken courses about specific businesses' general attitude and whether they just seem concerned about processing as many students in as short a time as possible – you need to know that any problems you may encounter while training will be taken seriously. Another consideration is whether you ever plan to dive again: if this seems unlikely, resort dives (a single dive with an instructor) will set you back only $75 or so, and they're usually available on day-trips to the Reef and island resorts. Qualified divers can save on rental costs by bringing some gear along; tanks and weightbelts are covered in dive packages but anything else is extra. You need an alternative air source, timer, C-card and log book to dive in Queensland (the last is often ignored, but some places insist, especially for deep or night-time dives).

Snorkelling is a good alternative to diving: you can pick up the basics in five minutes and with a little practice the only thing you sacrifice is the extended dive time that a tank allows. If you think you'll do a fair amount, buy your own mask and snorkel – they're not dramatically expensive – as rental gear nearly always leaks. Look for a silicone rubber and toughened glass mask and ask the shop staff to show you how to find a good fit. If getting wet just isn't for you, try glass-bottomed boats or "subs", which can still turn up everything from sharks to oysters.

Stories of shark attacks, savage octopuses and giant clams all make good press, but are mostly the stuff of fiction. However, there are a few things at the Reef capable of putting a dampener on your holiday, and it makes sense to be careful. The best protection is simply to look and not touch, as nothing is actively out to harm you.

Seasickness and sunburn are the two most common problems to afflict visitors to the Reef, so take precautions. Coral and shell cuts become badly infected if not treated immediately by removing any fragments and dousing with antiseptic. Some corals can also give you a nasty sting, but this is more a warning to keep away in future than something to worry about seriously. Animals to avoid tend to be small. Some dangerous jellyfish are found at the Reef during summer – wear a protective Lycra "stinger suit" or full wetsuit with hood. Conical cone shells are home to a fish-eating snail armed with a poisonous barb which has caused fatalities. Don't pick them up: there is no "safe" end to hold them. Similarly, the shy, small, blue-ringed octopus has a fatal bite and should never be handled. Stonefish are camouflaged so that they're almost impossible to distinguish from a rock or lump of coral. They spend their days immobile, protected from attack by a series of poisonous spines along their back. If you tread on one, you'll end up in hospital – an excellent argument against reef-walking. Of the larger animals, rays are flattened fish with a sharp tail-spine capable of causing deep wounds – don't swim close over sandy floors where they hide. At the Reef, the most commonly encountered sharks are the black-tip and white-tip varieties, and the bottom-dwelling, aptly named carpet shark, or wobbegong – all of these are inoffensive unless hassled.

The Marine Parks Authority levies a fee commonly referred to as reef tax (currently $6 per person per day, though some tour operators add $5 extra for administration) to help fund monitoring and management of human impact on the Reef. On most tours and boat trips, you will be required to pay the reef tax in addition to the cost of the tour. You may feel a little annoyed at having to fork out the extra money, especially if you've already paid quite a lot for your trip, but this is simply a "user-pays" system to help ensure that the Reef is maintained for everyone to experience and enjoy.