This is a great combination! The rules for casting two spells with a bonus action and action simply state (PHB, 202):
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
In your example, you have used your Bonus Action via Metamagic to cast Wall of Water. There is no save for wall of water; it only creates battlefield control aspects within its area.
Should you cast it on a space containing a creature, that creature would be within the Wall (although do note the Wall is only 1' thick.) Also note that only the 5' section of the wall they are in (see below) is turned to ice (and if destroyed, does not refill with water.)
The creature is now in 1' of water and the follow-up Action Cantrip of Ray of Frost interacts beautifully with Wall of Water's cold damage response:
Spells that deal cold damage that pass through the wall cause the area of the wall they pass through to freeze solid (at least a 5-foot-square section is frozen). Each 5-foot-square frozen section has AC 5 and 15 hit points.
You'd now have a Wall of Water with a frozen section containing a creature in their 5' space (but only 1' thick of ice.)
But what can a frozen creature do?
This is going to likely get table-dependent. There are no rules with regard to being 'in ice' and what conditions that imposes (like Restrained or Grappled.)
How I'd rule
I'd likely give a Dexterity save to avoid the ice (DC set by the caster, much like with Wall of Stone trying to entrap someone) and then upon failure give them the Restrained condition.