YOUR COVERAGE
You can't really deal with Steel types. At all. Raikou's quick, but he won't be able to OHKO all the time, even with Thunder, and will likely be EQ'd in the face. You can't use Fire attacks nearly as well with Rain Dance up, so you need Ground attacks for Steel types. Invest more in Rock attacks for Bug and Ice types, and Bug attacks for Grass types.
YOUR SPREADS
Why don't your walls have any HP investment? That's the FIRST THING you max with a wall, no exceptions. Have you ever seen a Blissey with less than 714 HP? Srsly.
YOUR HOLD ITEMS
Wtf, Mystic Water on Kingdra? Magnet on Raikou? No. Fix that.
Now, onto the Pokemon themselves...
KINGDRA
Kingdra, as Ninja said, oughta be Modest. It's good on Rain Dance teams, and makes a good lead because it only has so many weaknesses, and if they try to switch in a Garchomp or Flygon or something to deal with it, you'll have just set up Rain Dance and would outrun the bitch with Ice Beam. BUT, I'd say invest 128 EVs into Spd, assuming you've already got 31 IVs, too, so that you can still outrun Yanmega leads after Rain Dance and their first Speed Boost. Ice Beam WILL OHKO. I can't criticize this much, since it's the first Pokemon on the team. Give it Leftovers or an Expert Belt though... It's got relatively nice survivability, and Leftovers helps that out some. Expert Belt pretty much makes sure it OHKO's things.
MANTINE
Mantine, it looks, is your Sp.Wall, and a Toxistaller. Toxic and Brine makes a nice combination if you ask me, but this setup you have has no reliable HP regeneration, and no real stalling ability. Toxistaller doesn't have room for Rain Dance. Besides, as a Toxistaller, it needs something to deal with Steel types, and Nintendo's cruel trick of the day was giving Mantine EQ without giving it the stats to use it. Since this is a Rain Dance team, Fire attacks are a no-no, so you need a Sp.Wall that can Toxistall and EQ. I would probably recommend Quagsire (Stockpile or Amnesia). Lanturn might also be a good choice, since it resists both halves of BoltBeam, and has plenty of time to Amnesia against the likes of Jolteon and such. Sadly, no way to deal with Steel types, though. Better against Skarm if you've got some kinda Skarmophobia, though. Once you decide on a different Pokemon, you can repost your thoughts on THAT one, since whichever you pick it'll probably be a better choice than Mantine, and I'll give you my thoughts on that one. =P
LUDICOLO
Ludicolo makes a pretty good physical wall, and really, THIS one oughta be your Toxistaller with Toxic, Leech Seed, Rain Dance, and either Sub or Surf, with the Rain Dish ability and Leftovers, maxed in HP and Def. Sadly, again, no way of dealing with Steel types. Magnezone and Skarm murder it completely, and Metagross sometimes has Thunderpunch, which gives you a little more reason to use Lanturn or Quagsire. Once again, Lanturn in case of Skarm, Quagsire in case of anything else...
RAIKOU
Of course, something speedy with Thunder. It can OHKO Skarm, easy... And Water types... Well, under the Rain, with STAB, it can OHKO a lot of things with ease. A lot of Steel types besides Skarm will be packing EQ, though, as well as pretty much every Ground type you encounter. I would honestly give it Roar in place of Quick Attack for PHazing. BPer's cry when you Roar as soon as they BP. XD; Besides that, it's hard to go wrong with something so simple, aside from that choice of item... Try a Lum Berry, as Raikou may be the unfortunate target of some Bronzong's or Crobat's Hypnosis.
FLOATZEL
Floatzel seems fine to me, but I'd try looking into a CB Relicanth or a SubPunching Toxicroak. A CB Relicanth can bust up walls nicely. Rock Head keeps it safe from the recoil, or you could just make him as quick and powerful as you can with Swift Swim for a few REALLY devastating attacks at high speeds in the rain. On the negative side, if you choose to go with Quagsire or Swampert or something like that, you'll have another major Grass weakness. Toxicroak can be pretty nice in that case, as it can, with Dry Skin and Black Sludge, keep its HP fairly high while it Subs and Punches. Focus Punch will help cover Ice types you encounter (besides Froslass, which Raikou can deal with), and you can also give it X-Scissor for Grass types, and after that, maybe Bullet Punch or Sucker Punch for priority or Taunt to stop Pokemon from using moves like Trick Room, which will DESTROY your team.
SCIZOR
I've found Scizor to work well on pretty much any weather team. I'm not sure how well it would work here, though, as a Fire attack may still OHKO it if it's strong enough. An off-hand Fire attack, like Overheat from a Primeape or Fire Blast from a Garchomp, shouldn't be too threatening, though. I would also recommend that you maybe replace U-Turn with either Agility or Swords Dance. If you go with Agility, you can keep Iron Head, but with Swords Dance, I'd recommend Metal Claw, since with Technician, Metal claw is actually slightly stronger than Iron Head, and if you don't have the speed to get that Flinch-chance in there, then there's really no reason to have Iron Head at all. Scizor''s big positive point is that it'll take practically nothing from Grass type attacks directed at your team, which gives you yet another reason to have Quagsire on your team.
I might make another post to break it down a little more into what I think could be effective...