Actually you don’t need to worry about it if you have a BP_Underwater actor in your level and have both the and Swim Volumes set, you can even set it up so the swim volume follows your character around by enabling it in the BP_Underwater details panel. The BP_Underwater actor will automatically set the swim volume to the same level as the surface.
Just make sure these 2 are set and the Swim Volume extends far enough in your level (or enable the option to have it follow player)
On a side note, there has been a simple fix for a performance gain added to BP_Underwater to prevent it from performing things like the follow and visual updates on Tick unless the character is near the water, it will be in the next update (which will be out very soon!).