Are you sure that is what the glyph is meant for? Have you seen a post from Blizz stating such?
Although it is completely reasonable to look at it that way, it could just as easily be argued that the intention of the glyph is to offset the loss of mana regen on fights with little raid damage.
Seems to me like both could actual be the intent of the glyph.
1) To give us more regen on fights where the shield won't proc often.
2) To give those that would rather forgo the tedium of constant refreshing for higher regen on high raid damage fights for a static return instead.
I personally feel that Water Shield is fine when the raid damage isn't so intense that it's knocked off every 10 sec.
It gives us another form of actively keeping our regen up indirectly, since the loss of the shield means we generated mana through damage.
That being said I think that it could probably use a higher ICD on the procs to tone down the refreshing needed on high raid damage fights.
Also, with a static mana pool of 100k the proc on Water Shield seems like it could be a bit OP if the proc'ing on high raid damage fights isn't toned down.
With a proc of 3k at 90, even if the mana pool increased to 200k at 90, it would be a 4.5% mana return for one GCD and no CD.