If you can get the UFO organs, you're set, they're the tough ones.
As for the other mobs, pretty much PLD/Anything can solo them. Bring echo drops, as some like to hit you with silence, and remedies might be a good idea for the paralyze. Otherwise, you can chew up pretty much any of the Ul mobs aside from the UFOs, and some of the OM mobs as well.
Here's some specifics:
Hpemdes (aka 'Puppyfish', 'Goldfish', etc) will 'aggro' you in a sense, but they are not agressive. Meaning that, like Lycopodiums in WoTG areas, they will simply follow you around like puppies, until they lose interest and move on. Even if you flash/stun/provoke/bind etc them they will NOT attack. Only after you actually do damage to them will they become hostile. The weakest and easiest Sea mobs.
Xzomit (aka 'Squids') These are non-hostile, and will not aggro. They travel in packs of 3, which consist of a leader, or 'Mother', and two smaller 'Child' Xzomits. They DO link, but in a weird way... if you attack the lead Xzomit, the two child Xzomits will automatically link, no matter how far away they are. However, if you pick off one of the child Xzomits first, the 'Mother' Xzomit, like any good mother, will ignore the plight of her child and continue on blissfully unaware. It should be noted that the drop rate from the child Xzomits are horribly low, but the mother is 100% guaranteed to drop an organ. Usually the strategy is to pick off the two kids first, then kill the solo Mom.
Phuabo (aka 'Sharks') Phuabos lurk just under the surface of the frozen sea, until someone wanders too close to them. They then erupt to the surface and begin attacking. Phuabos are some of the strongest mobs aside from UFOs, but Ul'Phuabos are still easily soloable by a PLD/RDM or PLD/NIN, and probably other subjob combos. HOWEVER, there are a couple of caveats. First, bring echo drops, as they have a move that will silence you, and second, pull them to an area of dry land to fight them. When over the water, they are much stronger, and have access to additional nasty attacks. /RDM is actually a strongly recommended sub, as these things have a shock spikes move that can stun frustratingly frequently. Dispel, despite being a subbed spell, lands almost all the time and is quite helpful.
Aern (aka 'Angels') These are also non-hostile until provoked. Their job is undetermined until they are attacked, at which point they will 'grow' a weapon, and take on the abilities of a specific job. They are not partied, but they DO link. Again, hey have a silence move, so bring echo drops.
"One born of a dragon, bearing darkness and light, shall rise to the heavens over the still land. Bathing the moon in eternal light, he brings a promise to Mother Earth with bounty and grace."