D&D 5E (2014) DM Advice Wanted: Drowning a solo PC Encounter

[Tormyr]

My underwater house rule is that you lose 1 round of air for each action, bonus action, or hit you take. The higher Con PCs get extra time underwater and can do more, but 5 minutes is David Blaine level shenanigans. A high Con PC who is fighting underwater can easily lose half of their time through actions and taking damage.

 

[HalfAlbinoMidget]

A question for you: is this going to be fun? For the PC? And for the group? If the answer to any is no, then don't do it.

I'm certain of it else I wouldn't do it. I'm confident in my player and myself. While I want to make it seeming extremely dangerous, it's really not with all the safety nets (nearby NPCs, a waterbreathing plant (not mentioned earlier), shallow rocks to catch breath, and no immediate encounters afterwards so the treasure can be retrieved easily--aside from a lot of diving that is). My goal, besides fun, is to flavor it well and not make the safety features so obvious on initial description.