Well, his Continual Flame skullcap does come with a neck strap... which looks undignified, and also makes your life worse if some creature swoops out of a cave ceiling and tries to grab the Continual Flame. So Esta did not do up the neck strap inside the cave. If somebody had given Esta more than one and a half moments of notice, or told him about impending rescue from a foreign adventurer his brain thinks is Mariona, Esta might have done up the neck strap to greatly improve the ease of tracking him in water.
Maybe she's got Detect Magic? That would make him pretty easy to locate regardless. He has got various magic items on him, and his incredibly expensive back-of-head-band in particular stays on his head Just Because rather than for reasons associated with physics.
So Esta has not done up his neck strap in order to make it easier for Mariona to locate him.
He has instead done up the neck strap earlier, just in order to not lose the Continual Flame once he is underwater; and secured a few of his other items about himself, or put the heaviest ones (like his mace) into his wonderful weight-reducing Holding Bag, while he was trying to clamber uphill as quickly as possible. The point is, maybe-Mariona's minimal conveyance of information was suboptimal, this happened by luck rather than due to any skill of hers, and if this actually is Mariona, Esta will no doubt have words for her later while they are going through the after-action report. Words and pain.
Anyway! Let's consider the effect of water moving at 19 meters per second, impacting on a 6th-circle cleric.