Outdoor activities entail risks. Hiking, open water swimming, trail running, climbing, etc can be dangerous (even life-threatening) depending on conditions and above all your experience, ability and fitness. Be a cautious adventurer: increase the difficulty of your outings slowly and bring all recommended gear with you. I would suggest starting with easy hikes (here you have tons from number one English hiking web in HK), then bush walking, add some easy streams, later rock scrambling, basic climbing, finally coasteering.
In case there is any doubt, most routes in this web are not for newbies, but for people with solid experience and fitness. Some routes would be considered completely crazy and dangerous by, among others, local Gob. So when I grade difficulty is based on the following parameters, examples included.
In Hong Kong some people say that they “hike” on HK Gob designated “Family walks”. Those would not be even graded here… Zero difficulty. I don’t consider the Peak walk or similar concrete routes hiking, as I would not consider a long march in the city hiking, no matter how long it is. Difficulty definition in this site is mainly driven by the terrain, slope, remoteness and to a lesser extent total distance and elevation to be hiked.
1) Easy well-marked path with gentle slopes. Mainly compact dirt paths and some concrete. Sai Wan Tin to Tai Long Wan, loop around Shing Mun reservoir, etc.
3) Part of the route might require bush walking or hiking in rocky surfaces, although mostly you will be on “civilized” paths. Steeper but with no hands required. For example going up to High West.
10) Completely out-path hike, with little to none regular hiking activity around. Tons of bush-walking and slow complicated navigation. Several treacherous or steep slopes where you need to use your hands. Loose terrain uphill or, even worse, downhill. Problems for those with vertigo. Longer, remote routes with no possible early exits or liquid/food replenishment. Therefore pack extra water, food, clothing, first aids, even consider a PLB for emergencies.
To a normal hiking you add the risk associated with doing it next to (or in) a watercourse. More slippery surfaces for which you should consider using specific shoes. You should try always to hike the streams uphill, otherwise added difficulty bonus.
1) You are going to be walking next to a stream, but the surface is quite even and rocks have good grip. No need to get your shoes wet. Shortish distance or with easy early exists. Clear navigation. For example, the stream from Shek O but going upwards.
5) Smoother rocks. Several points where you need to climb (use your hands) a bit. Most surely with ribbons in several sections, but with no obvious best path to hike first times doing it. Ping Nam if doing the whole route, several sections of Tai Shing stream.
10) Vertical climbs, difficult terrain either cause slippery or loose, long insecure sections, several walls to scramble down, 10m+ cliffs, etc. Climbing ropes, helmet & other security gear highly recommended. A machete or gardening scissors might be an option in case you need to find an early exit through the dense jungle. You are getting into really dangerous territory. Only for the most experienced adventurers. Kwun Yam waterfall downhill, may be the craziest thing done so far.
You need to be comfortable swimming open water which would entail, at least, being able to swim 200m & float in bit choppy sea for 5 minutes. Be cautious with the rocks: salty sea water can erode them and you can break them when pulling too hard. When climbing in the cliffs be aware of the risk. If you are good enough swimmer you can avoid most of the dangers cheating a bit and swimming bigger sections.
1) You will be walking on the shoreline but the rocks are even, beach sections, no cliffs to go up into, just some minimal scrambling. Swims, if required at all, are short and within sheltered shoreline with no waves but in the worst weather conditions. Several early easy exits.
5) You will definitely need to jump into the water and swim for some sections or climb quite high cliffs. Some sections can be really challenging if with big waves (1m+) and you might need to detour.
10) The route does not include almost any easy walking. You will be scrambling most of the time, with seriously difficult or long swimming sections. Remote and with no or very difficult early exits.
Long distance open water swimming capability is the obvious requirement. Navigation, mainly being able to go in straight line, is one of the most complicated things for pool swimmers. No much waves and currents in Hong Kong compared with other regions. Therefore difficulty is mainly driven by distance, intermediate stopping options, remoteness and possible exits. Go with another swimmer, ideally with a swim safety device and if going to a really remote area consider getting a Personal Locator Beacon to ask for help in an emergency.
All the previous is based on good weather for each activity. Namely, no rain, mild air temperature and warm enough sea water. An unsheltered level 5 dry hiking path can get way more dangerous in the middle of a super hot sunny day. Even the most simple coasteering route can get way more difficult with big waves. Consider it when planning and change plans in the go if the weather worsens.