I like the theory behind this idea but there is one problem. For this to work you already have to be standing on water.....which begs the question, what is the point? And if you cast it in a direction i.e. 'cast 'frozenwater' north, that seems like alot more work than just buying a boat.

A more useful riff on this idea imho would be that anyone using a boat for their sole source of movement across water OR a watershifter who isn't flying cannot enter a room that has 'frozenwater' because they run aground.

Make it so that this spell like quicksand, cyclone, etc. is a one use per every so hours type of thing and I believe you have a very nice niche use for it. Maybe call it 'glacier' or something.

EDITED TO ADD:

Assuming in a real fight a person is using a boat and flying or is a water shifter and flying and you dispel their flight, then I would make it really hard to get off the glacier (not impossible, maybe on the level of difficulty that ensues when you get caught in that caverndweller thing that makes it hard to flee a room)

Just a thought.

I was originally thinkin of it as a temporary foot/boot enchantment kinda thing, but I like your spin on it.

Also I like the premise that it gives an option for an Invoker that got black jacked and had their boat stolen that isn't teleport/word.

Nt