Thalassophobia aside, Vylett would enjoy Subnautica. Lots of looting and much better stick-and-rock survival crafting than most games with that junk as their main hook.
But I don't think it gets scary for most people until many hours in, or if you decide to go to deep water for no reason. The oxygen mechanic gates you from doing that.
Factoring thalassophobia back in, a stream like the Dead Space one would be the best-case scenario, where she isn't really scared by it at all the whole time.
But as long as it makes money for charity, then I guess it doesn't really matter if that particular stream goes as intended.
It's also a game that has a story and is easily spoiled, but I haven't spoiled it here, for the record.