Aquatic campaign deep diving!

Really glad you built your group-- Any way you can get to play is worthwhile in my opinion.

If you're just going for flavor, it's pretty easy to make up a mechanic like you have-- in exchange for one 5th level spell slot, both players get to travel underwater and the adventure mechanics are easy-. I imagine them swimming like Mario who, you'll note, never actually needed to breath in the old games. Getting back to your game, you handicap the wizard player one of his valuable spell slots to serve a narrative purpose.

If you put the effect into a wondrous item, now you've added a mechanic with a dimension. Imagine a torch-- everyone needs to see, but only one player needs to hold the torch. You have set it up so only the wizard has one hand tied (or one 5th level slot in this case).  If the effect is confined to a static item, like a torch of coral or some other poetic fantasy thing, then someone needs to maintain the device in hand, or in a worn item slot, or attunement, etc. It makes it such that the players have better agency over which takes the handicap.

In many cases, when party needs to have an aerial adventure the DM provides a magic carpet. It's a standard fantasy item to let the party adventure in a new environment, but it has certain limitations; someone needs to take action to direct its motion, and it's a static platform that the PCs cannot easily leave during an encounter. From a mechanical-narrative perspective, you are using a constraint on the players capabilities in the scene mechanically, and so creating some opportunity for narrative tension.

I think you would be well served to use the description of the spell i mentioned above-- I think it can be easily adapted to your narrative purpose and provides existing balanced mechanics--

This spell turns normal water (or watery solutions) into a frothy substance that is as breathable as air. Both air-breathing and water-breathing creatures within the area can breathe normally. Moving through airy water is easier than moving through normal water.
Swimmers make a DC 10 Swim check once per round; they swim at their normal  speed on a success, or half their normal speed on a failure. Creatures with a swim speed can simply use their swim speed without penalty. Creatures can instead use their land speed to move along the bottom at their normal speed. Creatures walking on the bottom do not pay any movement penalty for walking on sand, stone, or pebble  seafloor and only pay 2 squares of movement per square of muck or mud entered (see Marine Dungeon Terrain on page 21).
Finally, airy water negates underwater combat penalties for all melee attacks, although ranged attacks still take the normal underwater penalty. The spell does not filter or  purify solid particles, so casting airy water on very turbid liquids, for example, would produce an area filled with dust or smoke (and impose similar penalties).
Material Component: A small handful of alkaline salts.

I think putting the above effect into a bell or torch that has to be held by a diver adds an interesting dimension to encounters. Your PCs would need to remain within the sphere of effect or face ill effects of the deep ocean. It would be an issue if the object went missing or was knocked out of hand. There could be an interesting trade from one PC to the other to share breathing time while keeping good range. There's a lot you could do, and the above provides some mechanically sound and consistent rules to do it.

In this case, just adapt the effect that makes your scene run most easily and give it the constraint you find most interesting. So long as your are homebrewing the effects and rules of the spell, make it serve your narrative purpose as much as you can imagine. Currently, there is a dicey situation at the end of the day when the spell must be renewed. With this effect, you create a mini-crisis to continue adventuring, so use that tension to some good end.