I have some Mermaids, who seduce men and take them away to drown.
I want to give these Mermaids a Power to make their victims not notice the negative status of being under the cold, cold water, and the sensation of drowning.
How would I desgn this Power? MuIm?
Your mermaids are Faeries, right? If so, what kind of Vitality do they feed on?
Feeding on the Vitality of people not noticing what finally happens to them is hardly Faerie, I reckon.
I would use the RoP:F p.74 Glanconer (Kelpie) stats - with Allure and Steal Judgement - but keep drowning for real and thus also give the besotted lover a final chance to break free.
EDIT: Lady Wakasa in Ugetsu Monogatari (Machiko Kyo) inviting a gifted stranger (Masayuki Mori) to her home to marry him, and that stranger's terror when he recognizes that he has fallen to ghosts - that is a glanconer's ploy.
I wouldnt sweat it too much.
MuIm, MuMe, CrMe, ReMe, have your pick, the list is not exhaustive.
I would give them the relevant power-virtue (depending on how powerful an effect you feel like it is) and just give it something like the following text:
The target of this power is unable to sense the sensation of drowning and hypothermia from being in cold water.
The troupe is currently in the lower part of the English channel, so I am being inspired with sea-morgens
Mermaids could be either faeries or magic realm. If they are faeries then glamour alone should allow them to "safely" escort them to the bottom of the channel, where they are released from the glamour. A similar effect (lungs of the fish, etc.) can work for magic with concentration duration.