Well, we need to consider the how the production of different elements works.
If this is in a mono-water deck, it is producing water, but costs to start, and is dependent on creatures.
If this is in a water-something duo, it produces both elements (probably) but to a small degree, and requires water upkeep, which, in a duo, is harder.
That is worsened for a trio.
In a rainbow, you have some creatures from each deck, and after those creatures are played, that element becomes much less important to generate.
Thus, I think even if we can expect more than 5 monsters, this would be a fair cost/upkeep.