This Kelpie (3pp) in Pathfinder could be interpreted in two (very very) different power levels. One would be that the person in question is so infatuated with the Kelpie that they do not attempt to hold their breath, meaning that they will die in three rounds if the Kelpie has not been dealt with and they cannot escape the grapple (which they wouldn't be resisting, and thus wouldn't be escaping. )
The other interpretation would be that they hold their breath, which makes the kelpie a near useless monster due to requiring 30+ rounds to kill a PC.
Kelpie rule in question:
Once per day, a kelpie can produce an effect identical to the charm monster spell (caster level 9th). The target can make a DC 17 Will save to avoid the effects. (...)
The kelpie can automatically grapple a charmed foe and attempts to drown the victim. A charmed foe does not resist the kelpie’s embrace, and the drowning attempt does not allow an additional save. (...)
Drowning Any character can hold her breath for a number of rounds equal to twice her Constitution score. If a character takes a standard or full-round action, the remaining duration that the character can hold her breath is reduced by 1 round. After this period of time, the character must make a DC 10 Constitution check every round in order to continue holding her breath. Each round, the DC increases by 1.
When the character finally fails her Constitution check, she begins to drown. In the first round, she falls unconscious (0 hp). In the following round, she drops to –1 hit points and is dying. In the third round, she drowns.
Unconscious characters must begin making Constitution checks immediately upon being submerged (or upon becoming unconscious if the character was conscious when submerged). Once she fails one of these checks, she immediately drops to –1 (or loses 1 additional hit point, if her total is below –1). On the following round, she drowns.(...)
Given a comparison with the Harpy, who can charm any (or all) members of the party at the same time and take full round attacks at them without retaliation; she could potentially be downing a character in a couple rounds without too much trouble and doesn't require special terrain to do it. This would make me lean towards the more powerful version of the Kelpie.
Thank you for any thoughts or rules arguments that would help me figure out which interpretation makes more sense.