High-five. I have 2 of those. Hades from earlier and Europa from yesterday. Not really sure how to build a water grid beyond grabbing a bunch of daggers, and I don't plan on buying Gislas either. Maybe I can grind out 25 more gold moons to get my first.
Actually, so I have Hades, Zeus, Europa, and 1* Baha. I also have 2 sunlight stones. Who is most worth dropping those stones into? Or should I just hold them for now and see what happens?
As deathkiller said, FLBing Baha is probably for the best. Do check out the FLB materials beforehand, in case you decide the grind is too much.
As for your water grid. Magna water is a mess, I wouldn't go there with Europa, not even worth considering. In case you do decide you'd rather try your luck at it, get 5 daggers, fill the gaps with RoB axe, Baha weapon, 2 Unkown, main hand.
Using 1 unkown instead of 2 allows for better cosmos synergy (unless you have a FLB Pinya knife or xeno cocy's is a dagger), but come Xeno Cocy farming 2 will probably give better results (it's also harder). I'd probably knock either 1 dagger or 1 unkown to make room for gabu wand, unless you are using xeno as MH.
If you don't go water magna, though, things get better. One Murgleis is basically a must when mix-maxing, so I hope you have SSR Kat (Murgleis could be used in magna if you don't mind spending the resources), but it is not necessary.
Primal water builds can largely skip Auberons/Gleipnirs nowadays, if you use Ely and have Murgleis, but you should consider getting them if you want to play something else, as your only buffer will be 5* Quattre unless you got Diantha/Christmas Arulu lately. In other words, use Chev Axes as filler and work towards getting Wilhelms and Fimbuls, next time Fenrir shows up.
Usually having 1-2 Fimbuls is enough, but having more is good, too. You'll want your RoB Shellfists and Axe too, and 1-2 unkowns (crunch some numbers on motocal, once you have the time/know which weapons you can reliably get) and SSR Gabu wand.
Using 1-2 Omega daggers is probably a must, too, unless you whale with Durandals/Murgleis.
One of the things I like about primal water (my main grid) is that it seems to accept whatever you throw at it, so don't worry about your grid being in shambles at first - mine still is! While it's far from optimal, it works nonetheless. Murgleis is just broken like that.
This is my quick and dirty introduction to primal water, but I'm still trying to min-max my grid, so take it with a grain of salt and, as always, take into account wether you want to actually grind for a particular weapon or not before going for it. Sometimes getting +1000 damage might not be worth the grind if you aren't as hardcore.