Yea, it's very dependent on what your options are. Not only what water eidolons you have, but what else do you have. If you already have a bunch of 2k atk eidolons, then you'll probably need to wait until you pick up... Monoceros, I guess? (that's next year)
Then your sub slots would probably look like Tiamat/Tiamat/Monoceros/Jack Frost/Jormungandr.
It'll also depend on what you've put your +1's on. Maybe your water eidolons are better than base due to those +1s, then the tipping point shifts over in one direction. Or maybe your non-water eidolons are better than base, then the answer shifts in the other direction.