They're oozes, and they're aquatic. Not much to say here.
Probably the most confusing Fiend Folio monster in terms of mechanics, and that's saying something.
The bloodbloater is a 2 HD swarm of diminutive oozes. Every other fiend folio swarm got updated to properly work with 3.5's rules, but this one for some reason was skipped over, causing it to have a size of 'medium'. It doesn't break too much because of the way swarm traits are worded, but it's weird. Stats are okay, with a very notable constitution score of 22. Land speed is a pitiful 5 ft., swim speed a reasonable 30 ft.
Special abilities are fire vulnerability (ouch), Distraction, and blood drain. Unlike virtually every other blood drain in the game, this one drains strength rather than constitution. Is that how the ability worked in 3.0, or is this just another crazy trait?
All things considered, the bloodbloater offers potent defensive abilities and good bulk for a mere two HD, but suffers from the lack of limbs, speed, and advancement options (maybe consider dragonfire adept?). The spider swarm, another 2 HD diminutive creature, managed to get +0, but the bloodbloater seems like a downgrade compared to it. For now, I'll assign -0, with the caveat that in an aquatic campaign this is probably worth playing.
Have your players wizened up to the 'treasure floating down the corridor' trick? Do they poke the air in front of them with a 10-foot-pole, fearing the day they see its end corrode away? Surprise them with an invisible ooze where they'd least suspect it: in the middle of the ocean!
Flotsam oozes are 2 HD, and medium-sized. Physical stats are decent (most notably constitution), special stats are horrid. The flotsam's sole attack is a weak slam (1d6) that does have the neat rider of auto-grappling its target.
Additionally, any creature grappled by the ooze's slam becomes the target of a free extra slam on its next turn. I remember submitting a War Mind flotsam ooze for the VC once, who'd grapple thousands of tiny animals, get a ton of free attacks-that-target-two-squares, and use those to deal ridiculous damage versus most opponents.
Other traits include the ability to hold on to weapons that hit you (decent at low levels), and partial invisibility in water.
Also of note is that with just one HD of advancement, the ooze becomes Large, which boosts its reach and damage output by a considerable margin.
At the end of the day, the flotzam ooze is still an ooze, though. Horrid movement outside of water, low mental stats, a lack of limbs or slots, and no clear advancement options (grapple fighter? dragonfire adept? totemist?) all contribute towards my final rating of -0 LA. Again, not bad, and arguably good enough to warrant playing in an aquatic campaign.
Surprisingly interesting for a 5 HD ooze. It's huge-sized, with +10 or better to all physical stats, a 2d6 tentacle attack with bonus acid damage, cold immunity, surprisingly fast land and swim speeds (40 and 60 ft., respectively), and a great no-action mass-nauseate in Stench, which is even friendly!
It's got a poison ability that activates 'whenever it deals acid damage to a target'. The mental image of a reekmurk caster delivering poison through acid spells is certainly amusing (if hard to realize mechanically). Additionally, the reekmurk deals passive acid damage to all creatures in its reach, allowing for even more poison opportunities.
Of course, there's downsides. No limbs, low mental stats, incredibly punishing sunlight vulnerability, and 5 HD don't make for a great character, the passive acid tendrils are decidedly unfriendly, and advancement is once more unclear (acid-breathing dragonfire adept?). That said, the numbers on this are certainly big, and a 5th-level martial would have a hard time matching this.
In the end, I think I'll go for -0 anyway, with the now-familiar disclaimer that this could be completely viable in the right game.