Tidus is the new cool guy in town and obviously most of us that managed to get him are going to start searching for Tidus friends to try and chain with, but as I was doing my research I noticed this guy has some pretty nice synergies with other characters that would greatly improve his performance beyond just slapping a water weapon on all the DDs and going to town. So I think this little post could be useful for choosing your water based party and add your thoughts about the mentioned units / add others that I forgot about.

Rikku

The most logical companion to Tidus because 1) they're friends :), and 2) you're gonna have her in your party anyway so they better learn to work together. Rikku has a multihit water skill in tidal wave which can add damage to his chains, but more importantly, they're both characters that benefit from activating their LBs ASAP. This means activating Rikku's TM skill eccentrick now helps your party achieving both the coveted AOE reraise and a 75-100% water imperil.

Mercedes

Our resident pirate girl now has a very real chance to become useful in the party thanks to her great (albeit costly) enhancements. Tidus' main problem is not being able to make use of his water imperil on turn one since it's tied to his LB, but with Mercedes' fully enhanced sea king's chop you can add a whopping 75% water imperil right away, which is equal to Tidus' base LB imperil. On top of that, the same move turns her into a very decent finisher with a 350% dmg modifier that can make full use of the water weakness by equipping her own TM.

Mercedes geared up with a bowie knife and her axe plus the usual atk boosting kit can reach a scary 900 atk thanks to her 100% axe mastery enhancement, and she can further boost the whole party with a 3 turn 100% attack buff that cannot be dispeled. This skill overwrites the atk buff on Tidus' get pumped! and synergizes well with Rikku's Hyper NullAll, adding the damage boost to her mitigation for a well rounded power-up.

Zyrus

This GL exclusive wizard is a great team player despite his lone wolf portrayal and the fact that he's permanently giving you the eye. First, his gimmick also relies on a LB activation, which further optimizes the use of eccentrick on a Rikku/Tidus/Zyrus party. Second, his flare decreases water resistance by an underwhelming 30%, but does so on turn one if you so wish, which means Tidus' first attacks will at least get a bit of extra damage until you can trigger his LB.

But here comes the good part. If you can properly line up their LB activations, Zyrus' blood rend can be dual casted for a total of 1000% water nuke of a finisher that gets the benefit of Tidus' 75% imperil instead of flare's measly 30%! The only downside to this is that it takes at least three turns to set up, but in the meantime you'll still be doing decent damage as all the required steps are activated using damaging moves instead of simple buffs.

Rem

Wait what? Yeah, Rem can actually have great synergy with Tidus thanks to an interesting mechanic (oversight?) that was discovered by /u/Kindread21 in a previous post. Basically, Rem can equip a dagger together with Camille's TM sword and their dualwield granting restraints will somehow overlap each other and work things out. Using this trick and some L&As, Rem can reach her 300% mag cap for some great damage, even if it isn't her BiS. The small detail is that by doing this, you've suddenly turned her into a water player, since Camille's TM is a water element weapon.

As you might know, the nature of Rem's dagger boomerang makes it a magic attack that benefits from DW, killers and weapon element. It also happens to power up with each use, turning it into a freaking 1200% magic dmg skill in just 3 turns. At this point I don't need to tell you that this on a 75% water weakened enemy that just got quick-hitted is just plain cruel. But wait there's more! Rem also happens to be a great healer, so by using her you're covered with curajas, an esunaga skill that also cures, and an on demand single target reraise (which complements Rikku's LB on her charging turns) and through all of this REM won't lose her stacks. Woah.

Honorable mentions

Seabreeze Bikini Dark Fina with her water Distopia which can climb to 700% on the third turn. Great but not as good as the other monsters listed here.

Beach Time Fina can also deal a 50% water imperil right away and can do a decent healing job with a DC materia. She also gets entrust to further speed up that sweet Tidus LB if required, but the rest of her abilities are kinda sub par so she probably should be used with proper backup.

So that's it, I hope some of you can find this info useful in building your blitzball teams. Insert obligatory Tidus laugh here.