I greatly enjoy a swim or a surf, and the ocean habitat is full of some of the world's most diverse creatures. However, this vast habitat is also full of potential dangers. Avoiding injury is easy, and a person isn't particularly tasty fare for most Pokemon, but prevention is always the best thing.
Tentacool and Tentacruel
Tentacool and their evolutions are a sort of non-polyp Cnidarian(or a Medusa) that frequent ocean areas. On their own, they're not as dangerous to humans, but these jellyfish Pokemon have a nasty tendency to 'bloom', or swarm during a period of about nine to ten days after the full moon. When swarming, the 'orbs' on the sides of these jellies' heads are bio-luminescent, which makes them easy to see in the water.
Like other members of Medusozoa, these two species have a powerful sting caused by nematocysts, or stinging cells. Contact with a tentacle releases the barbed nematocysts into the skin, which release venom. While a single sting is unlikely to do much, a few people are allergic to the sort of venom produced by these Pokemon.
Carvanha and Sharpedo
Like other sharks, Sharpedo(and its preevolution, Carvanha), Sharpedo skins are designed to reduce drag in the water. Their skins are made up of placoid scales known as 'dermal denticles', which are almost exactly like their teeth. The scratches caused by their Rough Skin ability are nasty, but they can be avoided as their scales always point downward, towards the tail.
These two are apex predators, and while fatal attacks on humans are considerably rarer than the media would have you believe, both Pokemon are especially prone to eating almost anything they can find that doesn't eat them first.
Huntail and Gorebyss
Their preevolution is harmless, and both species are exceptionally rare in the wild, but these two are nonetheless dangerous. Truly wild Huntail and Gorebyss are only found in an area of Unova's Route 4, and even there they're incredibly hard to find.
Despite their considerable rareness, there is always the chance of invasive species.
Gorebyss, despite their beauty, are truly ruthless hunters that pierce prey and suck their blood, while the rougher-looking Huntail is an angler, preferring to lure fish in with bio luminescence and a tail shaped like a smaller fish.
Gyarados are probably the most publicized. While harmless as Magikarp, an evolved Gyarados is certainly a force to be reckoned with. As seen in the rather famous Lake of Rage incident, when one Gyarados is upset, the others seem to catch on through some nonverbal, nonphysical cue and begin to rampage as well. In this particular incident, the famous 'Red Gyarados' were the result of a group of underleveled Magikarp evolving. This appears to have caused them to become much more aggressive. Normally, Gyarados in the Lake of Rage are, for lack of a better word, calm(or at least at a low-seething boil) and only cause major damage when they're agitated. (Boy, do they. Luckily for me, the scar isn't that big..)
Frillish are opportunistic feeders. They generally prefer smaller prey, but if it gets too close, a Frillish isn't going to picky. They like to drag their prey down to the bottom(while still conscious!), and devour it there. There are no accurate reports of human fatalities, as the physical evidence is often eaten by scavengers before it can be properly identified.
Basculin come in two varieties. In most species, a different colored member will rarely be treated any differently by its own species. However, the two varieties of Basculin are 'mortal foes', so to speak, and are fiercely territorial. An invader in a Basculin's territory is absolutely verboten.
When around these predators, it's best to avoid wearing colors.
(Snerk. RED and BLU Basculin. Note to self: Remove opinions from my reports before publishing them..)
-Avoid swimming a few days after the full moon, when Tentacool and Tentacruel tend to bloom.
-Do not swim in fenced-off, prohibited areas. Signs are there for your protection.
-If stung, remove tentacles and leave the water immediately. Douse the area in water as hot as you can stand without burning yourself and remove any remaining nematocysts with gloves or tweezers. White vinegar has been reported to be successful at relieving the pain of the nematocysts.
-Avoid Sharpedo-infested waters. Not following warnings is the easiest way to get hurt. (I know from experience...)
-Swim in groups. Predators will often go after a lone individual.
-Wear non-flashy clothing.
-Don't provoke anything. A slow-swimming Sharpedo, Carvanha or Basculin is gentle, and will often only approach swimmers out of curiosity.
-Above all: Keep your Pokemon with you.