Sorry you were nailed by a Portuguese, aka bluebottles, they do hurt. Been hit more times than I can remember, even got put in the hospital once as a teen with a major overdose of venom. I don't recall the scaring being permanent, usually goes away.
Thing is, just like sharks, your best protection is understanding their "habits." The second best protection for Portuguese is neoprene in the form of a wetsuit. A knife will be of no use whatsoever, as with many (most?), other forms of normal marine life except for perhaps gently pulling tentacles off your skin once you're back on shore.
Portuguese man-o-war are at the mercy of the wind, where it blows, they follow along with current transport. In my area of SE Florida, they come from deepwater out in the Florida Current/Gulfstream to the east. If the wind is NE to E in winter you tend to see them more nearshore. They are rare in the warmer months. They can be blown and carried by current into sheltered bays, like Biscayne Bay in Miami. They can occur over vast areas, typically tropical but are found in temperate areas of the Atlantic like the Med, around the UK. They also occur in parts of the Indian and Pacific Oceans.
The larger the float or bubble, the longer the tentacles. Wikipedia claims tentacle lengths up to 165 ft.!!!? I thought they maxed around 50 ft.. Anyway, you can be swimming along the bottom when literally out of the blue one of these buggers can nail you, across the face, arm, wherever. The tentacles contain small nematocysts. Nematocysts look like little harpoons, they fire embed in the fish target, or us, and inject some very painful neurotoxin. The toxin is intended to paralyze prey. For each inch of tentacle, there are a bunch of these harpoons. The larger the float, the longer the tentacle, the longer and deeper the nematocysts fire. They tie the tentacle to you and everytime the tentacle touches you, more harpoons are fired into you carrying toxin. If you struggle are washed by breaking waves it is easy to get a nasty sting. The tentacles/nematocyts stay viable even if broken or on land for an extended period of time, they still can hurt. Thin sensitive skin, your face, between your fingers, toes, arms, etc. are more vulnerable than calloused areas like the heels of your feet.
The trick with man-o-war is to know when and why they occur where you kite. In Portuguese season here (usually September through April or so), I am in the habit of looking along the beach and the seaweed line and in the shallows nearshore. If you see a bunch of fresh Portuguese there on the sand, good chance there are more where those came from floating in. If you see a lot or if you are particularly sensitive to them, best wear a full wetsuit, booties too might help. You can go in your birthday suit and there is nothing to say you'll be hit by any. The odds are just greater, any exposed skin will get harpooned, thick enough neoprene should deflect the harpoons. You always should be aware of your surroundings when kiting, right? One more thing to look for are Portuguese. If you're in the water, keep looking to windward to see if floats are heading your way. Sometimes they can move pretty fast too. Remember the tentacles stream out into the wind or current carrying them. If you are going to fall, try to do it well upwind, better still well past the float but not close by and downwind or current of it. Seaweed mats can be laced with active tentacles. If your kite goes down and portuguese sweep into your lines, they may well bind to the lines giving a nice task taking all that stinging stuff off later.
Usually, they aren't health threatening, UNLESS you get a major dose of toxin, and/or suffer an allergic reaction. Had a boat turn around during the kite race to Bimini from Ft. Lauderdale when one kiter suffered a severe reaction to a fairly minor hit on his ankle. Usually they are just painful, sometimes they can be a lot worse demanding prompt medical care. Understand putting pee and vinegar on the stings may have fallen into disfavor, go figure. See the link below. As a rule, they are fairly easy to avoid IF you pay close attention ... but, not always. Wetsuits can provide some easy "partial" insurance for lapses.
Tips on first aid for man-o-war stings appear at:http://www.aloha.com/~lifeguards/portugue.html
stefenalli, i agree with the knife. good against any wildlife. fell into a school of dirty yelly fish 3 months ago (toby called them man at war or so?) still have these scars. A lot of people do watersports in the Caribbean and get accidents as well. those numbers are not even registered whatsoever. only when it's fatal. as you said: use your common sense and take responsibility for yourself. i know what i wear.