Previously logged here: https://forum-en.gw2archive.eu/forum/support/bugs/ele-double-heal-bug-under-water-video/3075170
As this was posted about a year and a half ago (by another user) with neither response nor fix, I thought it appropriate to re-log with a more general discription.
I have found this bug easiest to reproduce using the Thief heal skill Withdraw. The video I took is in Sparkfly Fen. Other professions, skills, and locations are possible, as shown in the previous thread, but take considerably more coordination.
Note: Upon leaving the water, the heal skill used above water is on cooldown, whether a heal skill was used under water or not. This is not shown in the video.
Video link: https://www.youtube.com/watch?v=dxfS8305kmg