Disclaimer: I don't play a Shaman as my main, so I may be missing something, but:
Regen formula from Spirit in combat: Spirit * Sqrt(TotalIntellect) * 0.0083625
Regen formula from Spirit out of combat: Spirit * Sqrt(Intellect * 1.05 for BoK) * 0.016725
Regen formula from base mana (in and out of combat): BaseManaAt85/20
For Shaman, Base Mana at 85 is 23,430.00, so your base MP5 is 1,171.00.
Total In combat regen: 1,171.00 + (Spirit * Sqrt(Intellect * 1.05) * 0.0083625)
Total out of combat regen: 1,171.00 + (Spirit * Sqrt(Intellect * 1.05) * 0.016725)
Trinket: Darkmoon Card: Blue Dragon
Dropping Mana Tide Totem increases your regen up to your out of combat regen level.
Your average Shaman with 5000 Intellect and 2500 Spirit will increase:
In combat with Mana Tide: 1,171.00 + (4 * 2500 * Sqrt(5000 * 1.05) * 0.0083625) = 7230 mp5
Out of combat with Mana Tide: 1,171.00 + (4 * 2500 * Sqrt(5000 * 1.05) * 0.016725) = 12118 mp5
by 4888 mp5 during Mana Tide
So that's a 12118 mp5 increase for 15 seconds. Let's pretend it only procced once in an entire fight. Then you throw down mana tide. 4888 mp5 for 15 seconds is a mana return of 14664 mana. It comes out to then be around 14664 mana per 360 seconds, or 203.5 mp5. However, it's more likely than not you can do it twice in a fight, so doing it twice in a six-minute fight comes out to 29328 mana per 360 seconds or 407 mp5. And this is completely ignoring all the extra procs you'll get without mana tide up since it has no ICD.
It could be a reasonable trinket. Definitely not BiS, but still interesting to look into.