If a character wants to use water magic to short out an opponent's magic, this sounds like your choice of: placing a maneuver on the opponent (Magic shorted out) to be tagged/invoked as appropriate, orthe same, but on the scene (Indoor Monsoon, which could conceivably be used against any attempt at spellcasting), or a block vs spellcasting.  Any seem to me to be appropriate applications of water magic.

Here's yer quote:

It is possible to use more than one aspect on a single roll, but you can’t use the same aspect more than once on the same roll or action (even if you’ve re-rolled the dice, that’s still the same roll or action).

Regarding the balance of Sponsored Magic relative to Channeling + Ritual: I think that there seems to be a common tendency to ignore the fact that Sponsored Magic is intended to be on a leash that is not entirely under the control of the character.  Casting a particular spell via Channeling has a predicatable effect every time.  You might fail your roll, you might be blocked, etc, but it's just the character vs. the mechanics.  If you use Unseelie Magic, however, it could fail because it goes against Winter's interests ... or because Winter simply isn't interested in the spell you are trying to cast.  Or because you unknowingly annoyed the Queen last century and she wanted to make you look silly.  This is touched on briefly in the power writeup on YS183 ("Invariably, these sources of power have some kind of agenda of their own"), and mentioned again on YS287 ("the spell you’re casting must align with the agenda of the sponsor"), and descussed in more detail on YS289.

If your GM ignores this inherent limitation in Sponsored Magic then of course it's going to be superior to the alternative, every time.  Or if you have a pushover sponsor whose agenda basically boils down to "whatever the caster wants".  Or if you build a custom power that is modeled off Sponsored Magic (as opposed to the standard magics + additional refresh for advantages), but with no agenda at all.

Well...he's been playing with a variety of names, so I played it safe. Thanks for the help though. Another player went the route of Ferromancer...technically all metal, but Mettallomancer sounds funny.

You could go with Allomancer...

A rubber ducky.  Yellow.

All assessment efforts require the use of a significant chunk of time, usually indicated in the skill write-up. However, this time invested in preparation allows these skills to come to bear in more time pressured environments (like a fight) where they would not typically be useful.

As UmbraLux mentioned, I think that what you are looking for is a Declaration, instead.  Assessment is basically a mechanism that allows you to uncover a hint from the GM in the form of a generally "durable" aspect.  For example, a character might make spend a few hours to make a Contacts roll, resulting in the discovery that his opponent is In hock up to his eyeballs, which he might make good use of some time later, perhaps during a social conflict.  Declaration is what allows you to apply knowledge that your character "already knows" to place an aspect.  The example mentioned earlier regarding RCV bellies is a good example of this, which takes no time at all (you still get a full action without penalties, even though a roll was required to "place" the aspect).  You can also use a declaration to "create a scene element", as done in the example on YS116.

A single-use bonus aspect makes sense for situations like Harry/Morgan's.

If you want a more permanent arrangement (kind of a Knight-lite), then consider having the character add the Emissary template (or replacing, if the character was formerly pure mortal).  The character modifies his high concept (possibly by tacking ", Esquire of Winter" on the end of it), gains Marked By Power, and might have further opportunities to develop other powers.