Whilst I really wouldn't want to comment on the use of flashes on seahorses, having photographed the turtles at Abu Dabbab many times I can honestly say that they have never shown any indication of being bothered by flashes. In fact, apart from snorklers trying to ride them, I have never really seen them bothered by anything.
In terms of the dugong, I understand it that he is very rarely ever seen their any more and bearing in mind the mobs that surround him when he does, blaming strobes is a bit rich.
A group of us were at ELphinestone photographing the oceanics a couple of weeks ago and we certainly never saw any reaction to our flashes, nor were we told to avoid their use.
I would write to Emperor and tell them that if they do not welcome photographers or the use of flash guns they should make it clear so that photographer make exercise their judgement as to whether they should use their services.
Daniel